There's Paint.NET :) Yes that's right, almost 100% written in MANAGED CODE. I remember using this application when it was released as version 1 on my Windows XP. Although a novel idea at that time, the performance was very sluggish. Perhaps written without optimizing the .NET code. But now, with their latest release v2.72, I can say that it's blazingly fast.
Startup time from zero and uncached, was around 1 second. The entire experience is very slick now, from opening windows, dragging selection boxes and applying Effect filters. Works fine on my Vista RC2. I'm gonna be using this more and more to edit my icons, images etc.
There's a alpha build for their new Version 3.0 that you can try, but mind the dust.