Apple - Paint.NET alternative for Mac OS X

You can use builtin Preview.app for adding arrows, shapes, text, magnifying glass etc. It's not as lightweight as Skitch mentioned by @jherran, but does the job You need.

You should try Skitch. It does exactly what you want, take screenshot, put arrows, etc.

Get your point across with fewer words using annotation, shapes and sketches, so that your ideas become reality faster.

Pinta is a free, open source drawing/editing program modeled after Paint.NET. Its goal is to provide users with a simple yet powerful way to draw and manipulate images on Linux, Mac, and Windows.

http://pinta-project.com/