All the MMORPGS I'm aware of comes with a free month to try it with (or all the major ones anyway, AC (Asheron's Call), EQ (EverQuest), AO (Anarchy Online), I think UO (Ultima Online)).
There's the box price, and the games aren't playable offline, so you're still risking cash either way.
There's one, called Redmoon, that's free at the moment (beta of some sort), but it's EXTREMELY monotonous.
http://www.redmoononline.com
EDIT: MMORPGs=Massively Multiplayer Online Role Playing Games.
EverQuest regularly has 2000+ people on a server at a time, Asheron's Call is similar, Anarchy Online's goal is a world supporting 100k+ players.