I'd use FreeBSD. OpenSolaris is officially dead. Solaris isn't low cost for sure. FreeBSD gives you a high performing io/network stack plus extras from Solaris

I'd use FreeBSD. OpenSolaris is officially dead. Solaris isn't low cost for sure. FreeBSD gives you a high performing io/network stack plus extras from Solaris world, like DTrace & ZFS, plus a focus of maintainers on just these features you seem to need most. This is contrary to many Linux distros, even commercial ones like RedHat, where the focus seems to drift to different directions over time, as they are trying to please different groups of users/donators/customers. If you decide Linux anyway, I'd recommend Debian instead of CentOS. Contrary to common belief, RedHat/CentOS distros are "enterprise-oriented" maybe in the aspect of supporting typical Java development, but when it comes to production operations their enterpriseness does not necessarily shine, esp. when things break. YMMV. These recommendations change a lot if you must deploy Oracle databases on the machines. Well many people want to give their 2 cents in this thread; while this is excellent, I'd like to point I'm a sysadmin of various Unices since 15 years so I claim to know what I mean and not just promote personal prefs when saying something about OSes (if it was my personal bias I'd use Slackware forever). If you had further questions and wished to ask them privately, you'll find contact links on my profile.