Microsoft Research's Accelerator Project exposes a .NET library for performing parallel data processing using a computer's GPU.
Accelerator is a .NET 2.0 DLL that requires DirectX 9, included with Windows XP SP2, and a graphics card that supports Pixel Shader 2.0 or above. Graphics cards without hardware vertex processing, common on cheaper laptops, do not appear to work.
Accelerator is released under a non-commercial Shared Source license agreement. This means that while a developer can look at the source code, special permission is needed to actually use it in a production environment. Also, the source is not actually included in the installer or on the project's wiki page.
The current build can be downloaded from Microsoft Research. Note that the date on this page is incorrect. The build you get was actually uploaded in July of this year.