For companies which offer more then one MMO, they give you a discount on your monthly fee, and you get to play all of the games in their stable. Station Pass is something like $30ish USD, and you get to play Vanguard, EQ, EQ2, SWG, Planetside, PotBS and a bunch of lesser known games.

That makes it a fantastic bargain, but only because VG, EQ2 and PotBS are more recent, up-to-date titles. I don't think I'd pay for an EA pass to play DaoC, UO and (a game I Have NO interest in, personally) WAR. I DO like the idea of a "season ticket" though.