On the non-Linux front, Joyent's accelerators (on-demand scaling via virtualization) use OpenSolaris, with the virtualization being provided by grid containers (virtualized userspace on a shared kernel).

Joyent have a good reputation but I'm not a fan of Solaris ancient userspace tools - something as simple as checking the size of a disk is made complex by Solaris not bothering to include an 'h' option - so you have to count your terabytes of disk in kilobytes.

The Solaris filesystem layout is non-FHS and a rather odd mess, with binaries in var and etc, and variable files in usr.