What I like to work with is high-performance computing, numeric computation, security/cryptography and as many threads as possible - that's what makes the job fun for me, not just solving a problem (that's easy) but doing it efficiently and fast.
Personal projects include developing own mobile-first framework & advanced CMS counterpart based on Django & developing a reference game engine from scratch to learn Vulkan.
My main OS has been Linux since 2008, haven't looked back since (I use arch btw).