Depends on how you want to categorize things. I put them into the Proprietary or Open Source category, but some folks put them into the Windows or Unix category. There are Operating systems that don't fit either of the win/*nix categories though, but they are the most popular ones.

I think there's four...  AIX (IBM's), SCO, FreeBSD, and the HP one.  HPUX?  Can't' remember.  AFAIK, linux is based on FreeBSD.  I think OSX is too.

Twitter is like a message board at the speed of light. I initially thought it was stupid, but now I like it a lot. Very powerful, especially searching.

I don't think there are any; maybe convenience, but only initially. In multi user, if a user has limited access to system files (or none at all like in *nix based OS) then when something bad happens as a result of user behavior, really the only thing that will get trashed is that user's account. … Read more

It's the interface between you (and the software you want to run) and the hardware. The middle man, so to speak

Yes. .25 * 3 = .75