I’m not sure what a copy of UO will run you now. I got mine from a friend who didn’t play it anymore. I’m sure you could find the disks floating around somewhere used, maybe on ebay. Once you get them, you have to do a full install, including patches, but you don’t have to start an account…
Hmm…here’s a link that’ll give you a jump start: http://www.fekt.de/help/newbiefaq.html
Remember, the free shards are run by enthusiasts. They won’t be as heavily populated as UO’s official servers (in the brief time I played around with it, the most heavily populated one I saw had, maybe 40 or 50 people on it?) and the code isn’t quite as polished either. You also have to worry about things like the shard operator’s cat taking a whiz on the server and bringing it down temporarily. But, hey, it’s free. Sorta.