any sort of roguelike makes for fun possibilities, but are usually near impossible to win, but then there are some, SOME that are unwinnable (dwarf fortress im looking @ you)...very few of them are enhanced with multiplayer, but when you do make a good find, theres so much more FUN to be had
if you have a deeply lit imagination full of wonderous thoughts and whatnot, your best bet is to check out any variety of MUDS. most come free, but others you may have to pay a subscription for. theyre not all that bad though, you just have to find which one suits you the fairest!!1