There are quite a few different really good board games that mid twenty year old's would enjoy. You can play trivial pursuit, mad gab, scrabble, sorry, sequence, scattergories, opposite sex game, etc. It is all a matter of what kind of games you like.
For a crowd you can break into 3 or 4 teams, Cranium has something for everyone (combination trivia, pictionary, charades and play doh sculpting). Dominion is an empire-building game that should appeal to fans of Settlers. (I liked Dominion about 10 times as much, though.) Backstabby types will love Junta.
For wordy types, Taboo, Balderdash and (of course) Scrabble are awesome.
I'm a big fan of Cranium, always a good time with lots of laughs. Other favorites are Taboo, Rummikub & Dominoes. It depends on how many people you have & how interactive you want to be. To see what others are saying about these games & discover new games to try check out http://www.boardgameratings.com/