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I find FreeBSD stable. Altough coming from Redhat, it will be a little d\more difficult, and fuseterating to install. I about kicked my computer out the windows. However, read a little bit and it becomes easy to install when you get the basics. It's stable and fast and used by Yahoo!, and even Hotmail for a while. I like redhat, however becuase of the fedora thing, I am rethinking as well. Can't really blame them for wanting to make some money, however I'd hate to see them turninto another Sun.
Solaris is a great OS, just expensive cost wise (w/ hardware and everything). You can use the intel version, however I have heard mxed reviews and don't know much about it.
If you looksing for something safe and secure, and that has a prety large user base cmpartively speaking, try out FreeBSD