Out of all the games listed so far, I'd say only a few truly fit the OP's request for open ended.
Yes Baldur's Gate, Planescape, Fallout, et al. are more open ended than console RPG's. But they still have a main storyline that you must complete. You can't play seemingly infinitely. You'll quickly run into ruts where you've completed everything there is to do in every section of those games (esp NWN & Planescape - in fact, both of those games are much more limited than Fallout with fallout 2's ability to travel where you please).
All games are finite, but only a handful feel unending. Three of them come from Bethesda, and those are the Elder Scrolls games, Arena, Daggerfall & Morrowind; soon to add Oblivion to that list.
In each of these games you can wander aimlessly in any direction and find quests upon quests upon quests, along with tons of area to just explore and treasure raid dungeons, or speak with people and travel with NPC's (not too often but there are some). Even after you complete the main quest you still can go around doing whatever; they are truly the only RPG's I would call open ended.
Fallout 2 is a good choice though, lots of traveling, no real time limit. Fallou has a time-limit though. Take too long and your village will die, so that's not open at all (yeah you can toy with other things but I've played a couple times where I had to reload a save and rush through to make sure I could complete it).
Baldur's Gate II has so many quests that originate from Amn you'll feel like its infinite, it's really quite crazy. But still it has an end unfortuantely that finishes the game (Throne of Bhaal add on makes it even longer!

). BG is really quite short in relation to BG2, you can finish it in 20 hours and do most of what you could encounter, great game though.
Vampire The Masquerade, I would say is extremely limited. Esp for the first half of the game. You can't really choose where to go or what to do, you just do what is available. When you get to the other section of the game then you have a little freedom but it's still quite limited. VTM: Bloodlines is also quite limited, the game unfolds in chapters, there are side quests but nothing too extensive. You could find just as many side attractions in Final Fantasy.
Planescape, as I was saying above, quite limited. One of my favorite games of all time, wish they'd make a god damn sequel (and I hope every day that the modules for NWN will come to fruition, I'm not good enough at them unfortunately). But it's quite limited in where you can go and what you can do (not much backtracking ever either).
Best bet, grab Morrowind, Daggerfall & Arena are so old they're too frickin hard to get working on today's machines without a DOS emulator (I'm playing Daggerfall right now matter of fact) and I wouldn't wish to tell someone to play them because then I have to explain how to get them running

. One word of warning though, the amount of freedom you get in Morrowind, you might lose yoruself in all the side stuff to do and forget what you were supposed to do for the main quest. And hte journal entries will be so far back and mixed in that you'll forget about the main quest at all

(I know a few friends that don't even know what the main quest is ^_^ ). That much freedom is both a blessing and a curse. So if you do want to complete a main quest and have little reminders on occassion, any of the games I mentioned above give you some fun on the side but don't detract your from the quest too much (enough, but not too much more than Star Ocean Till the End of Time would).