I use Screenshot by Gik Soft in the Market. I like it because it allows you to set a delay in seconds before the picture is taken. I set it for 5-10 seconds, then navigate to the screen I want to take a screencap of, and wait to hear the click.
There are others that take the picture when you shake the phone, but sometimes I need my finger to be doing something on screen that I want to take a screencap of, and this is much easier than trying to hold your finger steady onscreen while shaking the phone.
Example: I use SwipePad to add a hidden launch panel accessible from any screen you are on. SwipePad works by making a small region in a corner a hotspot that you touch to open the hidden launch panel. This panel stays open till you select an app on it, or remove your finger from the screen. Trying to keep the panel open with a finger on it and shake the camera to take a screencap is almost impossible. Screenshot's timer is a perfect solution for this type of scenario.