I really liked playing D2 but my sister uninstalled it last night (and I broke the install discs a few months ago) -.-
I already tried Fate and Ultima Online
It can be single player or online I don't care.

My only requirement is that it's free or has a trial version.

supreme destene~?

Heard silverfall's like Diablo,it's B2P,single-player,but I think you can play online too,there's Titan Quest.Both game has a demo.I heard Dungeon Runners and Dragon Raja are petty much similar to Diablo too.
http://www.dungeonrnners.com/
http://www.newraja.com/

Try cronous? its D2 ish.. sign up ends at 17th. 2nd cb starts at 19th
http://global.cronous.com/web/news/board.asp?seq=24&bcode=1&category=2

Heard silverfall's like Diablo,it's B2P,single-player,but I think you can play online too,there's Titan Quest.Both game has a demo.I heard Dungeon Runners and Dragon Raja are petty much similar to Diablo too.
http://www.dungeonrnners.com/
http://www.newraja.com/

as Salyne said. Titan quest,siverfall and Dragon raja are pretty similiar to diablo. But there is some more thought.
Dungeon Siege 1 and 2
Shadow flare (http://www.shadowflare.us/) got a free "episode one" install, there are aproximating 4 episodes.
But ill advice you too buy a new Diablo II:p it dosen't cost that much nowdays