I've simply been using Solaris still on my old Ultra 40. It is finally starting to show it's age (I mean, it is probably close to 20 years old now) with 2GB RAM, dual 400MHz UltraSparc processors. The processing power is what you really feel the most, but it still does alright serving out my ZFS filesystem to the rest of my network (and acting as my own sftp server).
At one point I had it in my DMZ so I had access to my own net based storage from anywhere. I spent quite a bit a time hardening it, and setting up some spoof information (changed everything I could so that all the basic response information made it look like a Windows OS, only a detailed timing scanner/identify software would betray it was a Solaris network stack remotely, at which point my firewall would have already blocked their access before they could complete the scan). I had written a program that read the live firewall connection data which could write new block rules on the fly to ban IP addresses, subnets, and even all addresses assigned to that particular ISP (it looked up the whois information and kept track of number of attempts over periods of time and how many from the various subnet or ISP block to determine at what level to ban the IPs).
Anyway, it has my ZFS there...