Having fun with Haiku [haiku-os.org], the BeOS compatible operating system. It's very lightweight and blazing fast. Runs well in a VM. I'm currently porting a mini CLI app in C++ to it and despite the lack of some standard libraries, it's quite fun to do!
@Streakmachine It's already real fast in a VM, I guess it would be phenomenal on real hardware, ahah. This is an experimental OS and really not ready for daily use (imo), but it's so fun! And BeOS really had some interesting features.
@ericd I remember I experimented with BeOS 4.5 (I think) when they released for free for personal use, back in 2001. Was fascinating. Had some great ideas. Shame it died, but glad Haiku is getting somewhere.