Go and check the db, there are a lot to choose from.

There are a few custom audiogames which you've probably heard of such as Swamp, Humans vs robots, survive the wild and crazy party, as well as several audiogames that can be played online against other people, rs games, quentin C playroom, blind adrenalin (though that one is commercial), and Jungle.
Then there are some games that you can setup online games with other people such as sound rts, the vip gameszone games like galaxy ranger, super tennis and super football (those again are commercial),dragon pong and palice punchup.

Then there are a huge range of muds, browser games and mmorpgs for the pc, and Iphone.
In the muds category some have very extensive soundpacks, Alteraeon, Cosmic rage, six dragons, while many are just good games in text flexible survival, frandom, Erion, Avalon etc.

Then there are browser games such as core exiles, Drakor, puppet nightmares, Sryth, kingdom of loathing, metroplexities, monster breeder, torn city and god knows how many others.

Finally, there are games for Ios (and I believe android), where you can play against other people, turf wars, lords and knights, celtic tribes, the ucon games, most recently there is huboodle.

So lots of choice and I've not even covered a small number of games here, just browse around the forum and the db and see what takes your fancy.

swamp is always a good one and there's castaways 2 by the same developer. both games need a gamer account to play online but it's very cheep for what you get and the account covers both.