This is an (incomplete) list of my (favorite) operating systems

Mac OS X
By far the best OS I know (as well for noobs as for professionals and developers).
Just for gaming it is not optimal because there are not a lot of titles avilable.

Windows
Well, thi is definitely not my preferred OS. But I have to work with it. And with some modifications one can work with it (at least most of the time ;-))

Ubuntu Linux
The usability is just great. It's solid and fast. No other distribution was in that short time that successful on desktop machines. It is based on sid from Debian.

The newest version is even available as server version (and IBM certified).

CentOS
A popular Enterprise-class Linux distribution (besides Fedora) that is based on the RHEL4.
 
Gentoo Linux
My preferred distribution for servers and professional systems. You really exactly get what you need. It is very flexible and highly customizable.

Knoppix
The only live-CD that works without modifications with my external Firewire CD/DVD device on my notebook.

Damn Small Linux (DSL)
DSL is a very versatile 50MB mini desktop oriented Linux distribution. It has all teh main fuctions and can even run fully in RAM.

Other distributions
OpenSuse (Novell), Debian, ...

UNIX
Solaris 10 (very advanced) and OpenSolaris (open source) from Sun, OpenBSD (most secure), FreeBSD (performance & stability), DragonFlyBSD (based on FreeBSD 4.x), NetBSD (portability)

Minix
A rather painful experience. In the course operating systems we had to implement the nice command into Minix 2.0.
Minix 3 is now no more than just an OS for educational purposes. It is also built for resource-limited and embedded computers.

Amoeba
This is a distributed operating system. It runs on a processor pool that can be very heterogeneous. Several machines connected over a (fast) network operate as a single system. The system is very transparent. That means it hides the complexity of a distributed system from the users. The OS is very parallel and has a good performance.

OS for sensor nodes
TinyOS
Built for Mica Motes and programmed in nesC.

Mantis OS
It is more like a traditional OS with a classical system of multithreading. More flexible than TinyOS.

Contiki
The world's smallest full TCP/IP with a GUI