Skitch opens up a small app that shows the screenshot and allows you to manipulate it in a variety of ways: resize, rotate, annotate, etc. It’s ridiculously easy and lightweight, yet incredibly robust (in terms of quality of functionality, not necessarily breadth of functionality — it’s no Photoshop). It might be the most fluid annotation software I’ve ever seen.

I use Skitch on a daily, if not hourly basis. It forms the foundation for my screenshotting arsenal.

When I'm on computers without Skitch, it's like not having any eyes. Yeah, it's that bad.

When I am editing photos or album layouts with a detail-oriented client, Skitch is invaluable. I can make an edit, Skitch it and send it on chat.  I get my feedback immediately...make the clients edits, re-Skitch and in that way I get to an end-result MUCH faster than resizing for web, resaving, reposting and waiting for their response.