FreeBSD and OS X server are the only worthy server OSes in my opinion.
LULZ...
Our production servers are running CentOS heavily stripped down. Solaris is pretty good too, but I prefer the update cycle of Linux, so it'd be Debian for me.
The main reason to choose OpenSolaris over a Linux distro, for me, is ZFS, but you probably wouldn't see the benefits of ZFS on a webserver.