From Matthew Mastracci: I just released the third alpha version of my open-source .NET profiling application. It supports everything I've thrown at it so far (multi-threaded programs, NAnt, SWF apps) and is getting close to feature-complete.
It comes with a basic VS.NET add-in that lets you profile from DevStudio. There is a source package on the downloads page for those who are interested in tinkering.
Download it at http://nprof.sourceforge.net