iShowU is great if all you need to do is video capture (and cheaper). SnapzProX does much more than just video capture (but is more expensive).
Btw, if all you want to do is grab screenshots this is built into the OS and you don't need a programme for it:
Command-shift-3 will grab a screenshot of the whole screen.
Command-shift-4 will present crosshairs for dragging to grab a screenshot of area of the screen.
Command-shift-4 followed by spacebar will allow you to grab defined areas of the screen (inidividual windows, the dock, the menubar, etc.)
The above combos will drop an image file on your desktop. If you hold down the option key at the same time as the above, the image will be captured to the clipboard instead so that you can paste it elsewhere.