Free Tools I Like
Here is a list of some free, but not open source, tools that I like that make things a little easier in my development tasks.
ColorPixColorPix is a small simple color picker. You probably have used color pickers in other applications like Photoshop, where you hover the mouse pointer over a spot on the screen and it gives you the color value for the pixel underneath. But what if you wanted to be able to do that without having some design tool open? ColorPix does the trick for you.
Also if you are color challenged, like many good coders are, the makers of this tool have a lot of nice color schemes that you can use to dress up your applications in color sets that don't suck.
JustDecompileJustDecompile from Telerik is a free .Net decompiler. It can take a .Net assembly and give you back the source code, in either VB or C#. It has a very good desktop interface that gives you the assembly's source, and properly stacks up inherited classes so you can work your way into the base classes with ease.
It has been especially helpful to me in understanding what is really going on in the ASP.Net UI classes (e.g. List Controls).