Microchip's forum's great ressource. Hi-Tech C, CCS, MikroE, SourceBoost and all other Compiler brands have their own forum. Electro-tech-online is another but more about general electronics.

For C#, well MSDN forum is hard to beat, mix it with SourceForge, CodeGuru, Devx, Programmer's heaven, CodeProject and you'll cover most situations.

HTH