LICEcap is a slick cross-platform app that lets you create GIF screencasts.
When you load it, you’re presented with a resizable frame that you can adjust to fit the portion of your screen that you’re interested in recording:
Hitting the Record button gives you a few options:
Once you hit Save, it begins recording. When you’re done, you simply hit Stop (or Pause) and you have yourself a GIF: