This is one of the best gadgets I have added to my camera gear recently. I purchased it for just AUD$20 on eBay. There are two parts: the receiver (attached to the camera) and the remote transmitter.
On the receiver part itself, you can half-press to focus, and full-press to release the shutter. It works even without a battery!
The transmitter allows you to do the same focus-and-shoot, but wirelessly. This transmitter uses radio signal. Therefore you do not need to have line of sight to trigger the shutter. The signal works up to 100m and it is very responsive. The remote can also be used to control the shutter in bulb mode. You can also choose to activate the 3 sec timer on the remote itself, which is very useful for doing self portrait.
The remote really adds a lot of fun to shooting. I can leave my camera on the tripod, watch it in live view, and shoot whenever I want to without reaching for the camera. I have tried shooting with my camera tethered to the computer using Lightroom 3, and it works just as well!
Why I think this remote is really good.
My previous remote requires me to change between "normal" and "remote control" in the shutter release mode settings. This one does not require that (unless I want continuous shooting or bulb setting). When the receiver is plugged in, I can choose to trigger from the camera, or the receiver, or the transmitter without changing any setting on the camera. I really appreciate this kind of flexibility.