1 Linux (on x86, PPC and Sparc SS20; RH7.3 and mixed debian stable/testing)
2 Surfing, email, office, internet firewall, developing AROS, ... (basically everything)
3 Nope, I think it would be hard to find an OS that is so flexible (I have a headless firewall and a desktop machine all on linux) and where I have so much insight in what is happening on the computer.
Staf.