ZFS.
I have been a fan of Solaris since it went open. I used it before but I was just an admin. Now I am slowly becoming a zealot. Good, example; today I wanted to update the Sun Ray server to firefox 3.0.5 from SFW. I didn't want to mess up anyone else's surfing and wanted to be able to roll back with little to no hassle. ZFS to the rescue. I snapshotted the filesystem. zfs snapshot rpool/usr@today. This is too easy. I pfexec /usr/sbin/pkgadd -d firefox... I check firefox. It looks ok. I ask other users to try it and it doesn't work for them. I try to troubleshoot for a little while, I can't figure out why it only starts for half the users Its probably a permissions problem, but I don't have the time to figure it out soo.
zfs rollback rpool/usr@today and everything was working again. I need to find an easy way to do something similar with Linux. It is amazing when it just works.