Other Accessories
Once you start investing in equipment, things start to add up. Beyond the bags, filters, lenses, memory cards and flashes, there are tripods, straps, and more.
Battery Grip
The battery grip is a very handy extra to have, especially when you plan on being away from a power source for a long period of time or shooting a long event like a wedding. It gives you the flexibility of having two batteries and not having to worry about switching them out. The battery grip also has a nice feature that gives you a shutter button on the grip for easy access when you take vertical shots. They'll run you about $150 or so.
BG-E2 for the 20D:
BG-E3 for the XT:
Camera Straps
When you start to add heavy glass to the weight around your neck, the stock Canon strap just doesn't cut it any more. The more you take photos and walk around with your camera, the more likely you'll need a comfortable strap that distributes weight well and lightens the load a bit. I highly recommend the Op/Tech USA strap. It was definitely worth the $20 investment and has saved me avoid getting neck cramps.
Remote Controls
If you find yourself wishing you could take pictures remotely without using the timer, Canon has already made it easy with this wireless remote for your EOS camera. Only around $20 some dollars.
Canon Wireless Remote Control RC1
Underwater Housing
For those of you who enjoy scuba diving and wish you could capture photos of your undewater experience, you can. It will cost you a pretty penny, but you can pick up one of these underwater housings for your digital SLR for around $1200.
Ikelite Underwater Housing for the EOS 20D
Ikelite Underwater Housing for the Rebel XT