I second that. FRAPS is one of the most useful gaming tools that I've come across yet. It takes screenshots without fuss, doesn't hog RAM, and even allows you to take sceenshots automatically at intervals. There's also a video feature, which supposedly allows you to take 30-second videos, but I haven't tried that.

The free version takes pictures only in .bmp format, but it's just a simple matter of changing it to .png or .gif when you edit the screenshot.