There is not a good wireless release that works for the 20D that is really economical (the Canon one is around $400!). There are however plenty of wired ones that work very well.
The wired releases have a button that is just like the shutter release on the camera that you can half down to focus and then push all the way down to take the picture. Some releases also have a slide that can hold the shutter button all the way down for doing long exposures (so you dont have to sit and hold the button down).
I HIGHLY recommend the YN aftermarket shutter releases. They are much cheaper than the Canon official one (Canon RS-80N3 $50 vs. $4.80 for the YN) and really work just as well. Some of the other aftermarket ones are crap, the YN one is quite nice and you cant beat it for $4.80 shipped! If anything try it and see if you like it! You will not damage your camera with any of the switches as all they do is short pins on the camera. Only thing you could possibly do I guess is bend the pins on the camera if you tried to jam the connector in but you could do that with the Canon one as well.
Some of the Canon accessories are just stupidly priced, their remote switches are one of these items. It is about a 3 cent switch inside a plastic case with some wires, nothing more, nothing less. The canon might have a slightly better feel, but is it $45 more? I definitely think not.
Here is a link to the YN one at dealextreme which is used by many around here and other places. They are in Hong Kong so shipping can take a little while, but for the price who can complain!
http://www.dealextreme.com/details.dx/sku.8231