I'll keep updating this list as I find or remember more! :)
- GhostDoc - Helps you a lot documenting everything on your code.
- CodeRush - Powerful Visual Studio Add-in. I tried a lot of add-ins like this one but this is without a doubt the best of them. Powerful, very light weight, very easy to customize and enable/disable features... very good.
- Sizer - If you're a web developer this is a must have. It will let you size your windows to a resolution so you can test your web pages without needing to change the monitor resolution.
- ColorPic - Very good color picker with very good functionalities.
- Notepad++ - Best notepad around for developers!
- Paint.net - Very very very good drawing and image editor tool
Others "Nice to have"
- Quick Media Converter - Converts all your video and audio media to and from a myriad of formats.
- Greenshot - Very nice screenshot tool