There are literally millions of usable, vintage lenses out there that can be adapted and used on your EOS camera.
The Bob Atkins link gives you a list of mounts to look for, the ones highlighted in green are the most easily adapted, don't require any optical correction to make them work normally. Those are all 35mm SLR mounts, I believe. As a rule, rangefinder camera lenses will not be adaptable... such as Leica-M bayonet or LTM screwmount. Those have far to short a register or flange to film plane distance. However, Leica R lenses for their line of SLRs, are adaptable and fall within the "green zone" list.
It's the same with the SLR lenses.... some simply have too short register and are hard to adapt.... Optics would be necessary for them to focus to infinity. Canon FD are an example. Konica K/AR (Atkins incorrectly calls it Konica F) is another that can't easily be adapted for use on EOS. Also Fujica X and Minolta MD. Shame, there are some great lenses in all these lines (some can be used on four/thirds mount DSLRs and EVILs, though).
Another problem you can run into with some of the more extreme designs is that they might have elements that protrude too far into your camera and interfere with it's mirror. Retrofocus designs that tend to do this are usually only found on extremely wide angle lenses, though.
However, there also simply weren't as many truly wide lenses available in the "good old days" of film. Most film cameras were "full frame", so a 21 or 20mm lens was consider ultra wide. That's simply not very wide, on a crop sensor digital today.
Vintage zooms might not be worth the trouble. Many of them weren't particularly good in the 1960s and 70s. Much better quality zooms sort of coincided with the transition to auto focus lenses in the late 1980s and especially the 1990s. Unfortunately, a lot of auto focus lenses, even if found in some of the usable mounts, are not going to be as adaptable.
To be usable, the vintage lens must provide both manual focus and manual aperture control. Some lenses were designed for auto aperture control only, so don't even have an aperture ring (some Nikon E-series, for example).
On the vintage market, you'll find that lenses which can still be used on modern cameras... Nikon AI/AI-S for example... tend to hold their value really well and might not be a bargain.
You'll also find lenses that were offered with interchangeable mounts, which even then were designed for adaptability. An example is the Tamron Adaptall/Adaptall-2 series, which included some fine lenses. This is different from adapting a lens... There's not an adapter involved. Instead you change out the entire mount to fit it to a modern EOS camera. Not long ago I picked up a Tamron 90mm macro lens for very little money and ordered an EOS mount for it from China. Works fine. The lens already had a Nikon mount on it, which I kept so I can use it on some vintage Nikon cameras. I also have Adaptall-2 mounts for vintage Konica, Olympus and Pentax cameras.
An interchangeable mount lens can be adapted for use on EOS, no matter what mount it presently on it. For example, if the Tamron 90mm I bought happened to have a Minolta MD mount on it, it would just be a matter of swapping that for an EOS mount. It came with a Nikon AI/AI-S, so I actually had a choice... I could either swap that out entierly for an EOS Adaptall-2 mount (which I did), or I could have used a Nikon AI/AI-S to EOS adapter on it. Either would work.
To a lesser degree, Vivitar and Soligor offered some interchangeable mount lenses too.
Earlier there were "T-mount" lenses, too. These were the original interchangeable mount lenses, some dating back to the early 1960s. It's a simple screw on mount that became very universal. The T stands for Tamron... they invented that interchangeable system too... but they didn't protect their patent and left it open for any manufacturer to offer lenses utilizing the mount system. So there are literally millions of T-mount lenses available too... and T-mounts for EOS are common and cheap.
Third party manufacturers offered lenses in many different non-interchageable mounts, too. Some cheap alternatives to the OEM lenses and maybe not worth much, others quite good with cult followings still today. Those can be adapted just the same as the camera manufacturer's own lenses.
For example, Vivitar never made their own lenses (still don't, for that matter), but sub-contracted out to various manufacturers. Some of their best lenses were called "Series 1" and were make by Tokina and Kiron, among others. For a while they used serial numbers the first two digits of which indicated who the original manufacturer was. I recall Kiron made "22xxxxx" serial numbered Vivitar lenses for a while. One model I used and found excellent was a 24mm f2 (not a Series 1, either... so usually pretty inexpensive). Another was a 100mm macro made by Tokina. You can find more info about their serial number designations and old Vivitar lenses online.
If you stick with relatively well-known third party manufacturers, do some research and are fairly selective, there are some wonderful old lenses around that you can adapt.
Be aware that all will be fully manual... both focus and aperture control. That means they are slower to work with and as you stop them down your viewfinder will dim, making manual focus more difficult with a camera that already might not be very "manual focus friendly". Live View might help. Or tethered shooting. Or use an angle finder accessory with the viewfinder.
Adapters and interchangeable lens mounts (such as the Adaptall-2) come in two varieties: those with and those without a chip installed. Those with it, the chip matches up to the electronic contacts on your EOS camera, making it possible to use Focus Confirmation (camera needs to be set to One Shot in advance, though). This isn't auto focus, but it can help. Though when you stop down, much beyond f8 and Focus Confirmation also fails due to lack of light.
Both these shots were made with the Tamron SP 90mm f2.5 lens mounted on one of my 7Ds....
Tamron SP 90/2.5 macro lens at f11. EOS 7D at ISO 400, 1/400 shutter speed. Handheld, available light. Poppy buds
Tamron SP 90/2.5 macro lens at f11, with 25mm extension tube (close to 1:1 mag). EOS 7D at ISO 400, 1/400 shutter speed. Handheld, ambient light. Here's a shot of the lens on the camera....... And a shot of the lens off the camera, with the Nikon mount alongside...
That lens - with hood, Nikon mount, caps & the matched 2X teleconverter - cost me all of $20 US at a local secondhand store. The EOS Apaptall-2 mount with chip cost about $40, off eBay, including shipping from China. $60 invested... not bad for a lens as capable as this one. I've owned and used several other copies of this lens over the years, so I already knew it was very good when I found it in the store. (And it's versatile... currently has the Nikon mount back on it and is being used along with a couple other lenses to run a test roll of film through a Nikon FG I bought not long ago).
When buying these old lenses you'll need to watch out for possible problems. Use a flash light or a bright backlight to inspect them inside for any sign of fungus (I had to pass on a nice 24mm f2.8 Olympus Zuiko the other day... looked new outside, but was full of fungus). Manually operate the aperture and check that it forms a nice round or hexagonal or octagonal shape... sometimes a worn blade makes for an uneven opening. Also look for oil on the aperture blades, which can seep in from the focusing mechanism as the grease there degrades over time. Watch for dings in the filter mounting ring, that might indicate it's had a hard knock... might be okay or might have problems inside. Operate the focusing mechanism to make sure it's smooth and doesn't have any play. Look for scratches on front and rear elements... one or two small ones on the front will probably do no harm, but if you find any on the rear should, probably pass on the lens no matter how cheap it's offered. Inspect closely for scratches around screws or any signs the lens has been taken apart. That can be a red flag, might indicate an amateur repair. Look for useful accessories included, such as the matching lens hood, lens caps, cases,etc. Some lenses also came with tripod mounting rings and special filters. It's always easier and usually cheaper to get accessories with the lens.
If you are looking for some specific focal length, do some research to narrow down to some top candidates, then just start watching for those specifically. Don't get into bidding on them in auctions too quickly, watch a few come and go to see how much they sell for, and/or research previous sales.
Not saying you'll often find a screamingly good bargain like I did with the 90mm Tamron macro lens (among others, over the years)... But if you keep your eyes open there are literally millions of vintage lenses out there that can be adapted for a second life on your EOS camera... Sometimes for very little money! Just learn to recognize the more easily adaptable mounts mentioned on Bob Atkins website as well as what T-mount and Adaptall lenses with interchangeable mounts look like, then keep a watch for them.
