What is the easiest way to create multithreaded applications with C/C++?
There is no simple answer to this. It depends very heavily on what you hope to gain from multithreading, the platform/compiler, and which threading model you want to use. Every threading API has its pitfalls.
And just because no one has mentioned it so far, OpenMP is another option that is supported in many modern mainstream compilers and is intended to simplify the use of concurrency. http://openmp.org/wp/