Lately i have been learning to use IPlug by Cockos which is part of WDL: a fantastic library of handy C++ code, with a very generous license. IPlug allows you to build cross platform VST plugins (and AU on Mac) from the same codebase. It is very simple to use (compared to the standard VST or AU SDKs, and the implementation for a fully working plugin is relatively small. Anyway, one of the first things I have done with it is to port autotalent to VST/AU.
Get it here: