You raise a good point: is there something like http://pinvoke.net/ for non-windows interop? I know that mono is wrapping things themselves, but on open wiki might help.
There's the [P/Invoke interop assistant](http://clrinterop.codeplex.com/) which can help you create a wrapper for C APIs, and the mono team have created [Cxxi](https://github.com/mono/cxxi), a binding generator for C++ APIs.