Right now I'm contributing to Microsoft's coreclr trying to get it to build and run on FreeBSD, because I believe having a wide choice of software and software platforms will indirectly aid the project.
If a new project you see doesn't work on your platform, find out why and submit a patch containing your fixes. That will both make it easier for people coming along later and raise awareness of the platform amongst developers.
For errors I don't immediately see a resolution for, I've created issues and usually that has either been picked up by the .NET-team itself or they've provided me enough feedback to work it out myself.
When submitting small patches nothing special is needed, but for bigger patches you need to sign a "contributer agreement". Which is done all online, automatically and in a matter of mouse-clicks.
The .NET-team has really done a good job here, especially considering how they said "they were really new to this (OSS thing), so please bear with us" when this whole thing was first announced.
This is mainly done and abstracted in a module called the Platform Abstraction Layer (PAL), which everyone else then relies on.
So far the biggest amount of work has been getting in the FreeBSD-specific things, and correcting the codepaths which incorrectly has assumed that any Unix not Darwin/OSX will behave exactly like Linux.
You know, ensuring stuff like '#include "gnu/somelib.h"' doesn't get included where it shouldn't, replacing enums with ints here and there. Stuff like that.
Except for one component, I've yet needed to genuinely write lots of code. It's mostly been following the build-process and see what fails.