Paint.NET is a cool drawing application written using the .NET framework (version 2.0 required). It's also open source, so you can grab the code here. It has a bunch of useful features like layers, good history / undo management and plenty of effects, in addition to a nice Photoshop-like user interface.
It is a little CPU intensive: try drawing a bunch of lines on the canvas, then apply a Gaussian blur with a 15 pixel radius. Theoretically, .NET should be caching the compiled bytecode so it should get faster after the first time, but I didn't notice that.
I played with an early version of this. I do recall that it was a bit sluggish. I'll have to give it another try.
Version 3 is out now. Requires .NET 3.0 which you can get from Windows Update.