One of the very first tools that I install on a rebuilt/new PC is Greenshot. From their website (emphasis mine):
Greenshot is a light-weight screenshot software tool for Windows with the following key features:
- Create complete or partial screenshots quickly.
- Easily annotate, highlight or obfuscate parts of the screenshot.
- Send the screenshot to a file, the clipboard, a printer or as e-mail attachment.
I find it really handy for taking screenshots, annotating them, and then pasting them into an email. Mine’s configured to automatically save the capture to a folder, as well as open it in the image editor. I can then make changes and re-save over the top of the already-captured image. 
A handy keyboard shortcut in the image editor is CTRL+SHIFT+C. This copies the modified screenshot with annotations, obfuscations, etc to the clipboard.
Where: http://getgreenshot.org/
License: GPL
Blog: http://getgreenshot.org/blog/
Twitter: http://twitter.com/#!/greenshot_tool
