Mac: Animated GIFs get your point across visually without using a video. You often make animated GIFS from an existing video, but if you just want to capture your screen, GifGrabber will do the trick.
GifGrabber is like previously featured Giffing Tool, but for Mac. After you start the program, draw a box on the screen and press record. You can also start and stop the capture with hotkeys, so you can avoid getting your mouse in the way. The program does some simple trimming and shrinking of the file, and you'll have a GIF to share wherever you want.
GifGrabber [iTunes App Store]