Free OS X solution for getting screenshots, cropping and annotating them

I am looking for a simple/fast, free solution that will allow me to get screenshots from different windows, crop areas that are interesting to me and adding some comments to them.

It's main use case would be to track UI changes in applications, so it would be a good idea to support layers.

On Windows I was using Paint.NET but this is not available on OS X. Also, do not recommend GIMP, it's free but not simple and loads really slowly.

Use Apple's built-in Grab.app to capture screenshots, Apple's built-in Preview.app to crop, and the open-source Inkscape to annotate the bitmap.

If you prefer an all-in-one solution to the Grab + Preview combination, try Skitch.
The lite version is free and should be enough for your -described- needs.