Snapshoter

Snapshoter is a tool that goes beyond just taking screenshots. Besides that basic feature, it lets you create reminders, collages with your images, and add watermarks, among other things.

The interface consists of a tiny sidebar to the left of the screen that lets you access all the features and see the latest snapshots you've taken in just one click.

The composition mode, maybe one of its oddest features, lets you create collages from all the screenshots you've taken up to that moment, apply different effects, and save the resulting image to use as you like.

Snapshoter is one of the most comprehensive screenshot applications you can find. And since it can sync with Dropbox, it makes it really easy to share your images over the Internet.