SpaceHeater is a project mainly written in C#, it's free.
Because I'm cold.
First off, this program could kill your computer - so use at your own discretion/risk.
This program will run useless computations on 75% of your logical processors, to generate extra heat.
The program will exit after 5 minutes, or when you press any key.
Works best with multicore machines.
SpaceHeater requires .NET 4.0 Client or Full profile, as well as either Visual Studio or csc.exe to build the solution.