We use SIGAR in the RHQ project. It's used by RHQ Agents to do process table queries and to collect various OS/system metrics. It's a great project, since it runs on all the major architectures and OSes, and has bindings/APIs for Java and a bunch of other high-level languages. Highly recommended.
Does a lot of stuff, but has many very inconvenient limitations, and VERY poor documentation. Stuff just won't work and you'll have no idea why until you dig up some forum post from 2006, since the documentation doesn't even give a hint at what the behavior is supposed to be :/
There are no 4 star reviews.