Joyent Love ZFS
Joyant, the people behind TextDrive and Strongspace are really keen on OpenSolaris and ZFS and actually use it in their production environment, and they're not afraid to advertise it either...
I don't use any Joyent products yet (but might when my current hosting plan runs out), but I must get myself one of these T-Shirts (I prefer the plain one).
Update: It's been a while since I wrote this post, but thanks to Jonathan Schwartz's recent post - Technology Adoption (2 of 4) - this post has suddenly become a lot more popular. I can confirm I am now running ALL of my sites on Joyent on OpenSolaris and I can say I'm very very impressed with the performance and reliability.