I recently bought a 24-105 and it's a terrific lens. The range is hugely useful; it makes a good one-lens solution for many types of photography, and the focusing is extremely fast. It can currently be found on the used market for around $700, and IMHO is a steal at that price. However, it's going to be a bit short for wildlife, and some might argue that you'll occasionally miss the wider end. If you do go this route, you're going to have to supplement the lens at some point with something wider, or longer, or both.
Another solution might be to keep your 18-55, which will do a fine job on landscapes, and supplement it with a longer zoom. Since you're considering the 24-105, I'm assuming that your budget is in the $700 to $800 range. At that price, on the used market, you're very close to being able to buy a 70-200 f4 IS, which is an outstanding lens and is usable for zoos, wildlife, etc., as well as for portraits and general photography.
Alternatively, as others have suggested, you could go for the 15-85, which is both wider and longer than your existing range, and will cover your needs except for wildlife. If you did this, and based on my assumption of your budget, you should also be able to afford a 55-250, which is a terrific lens for the price (under $200 on the used market) and will do a nice job on wildlife.
Good luck!