Portal and Portal 2.Most fun you can have with your clothes on.Loads of great humour,controls are the usual excellent Valve standard and the puzzles are great to solve.Also,there's going to be extra DLC on Wednesday so that should keep you busy for a while.Pretty much my 2 fave games of all time & I've yet to meet anybody who didn't enjoy it.
Edit:Just noticed Portal2 is $14.99 on Steam.As it was $46 when it came out,that's a pretty sweet saving.Portal is only 10 bucks so that's 2 awesome games for $25.While Portal 1 is strictly single player,Portal 2 has both single player and co-op,but don't let that put you off as the single player has always been the prime focus of the franchise.In Co-op you only play with one other person-a friend in the same room or somebody online.That's a shizzle-load of game for your bux.And there are virtually 1000s of free custom maps made by players of the game.(Avoid Portal Prelude that's a load of shite.)
EYE

ivine Cybermancy is a bit weird and a little unfocused,but highly original and unlike a lot of generic FPSes these days.You have this huuuuuge skill menu-including hacking,psi and duplicating a mini-army of yourself,you choose and change your weapons from a huge arsenal and the gameplay changes depending on the character's emotional state.It's almost a sandbox game,RPG AND a linear shooter all at the same time.It looks amazing at times,although some of its npcs are lazily designed & lipsync is non-existent.Not a game without flaws,by any means.Still,different enough from a lot of the shooter games out there so it's definitely worth a look.There is a multiplayer,but I've never played it.
The original Half Life.Don't let the antiquated graphics put you off,this is great fun.And unlike the sequels,you can actually shoot the friendly NPCs.Great fun to make a scientist run and scream like a little girl,"I don't want to die!".Of course the sequel Half Life 2 and its follow-up episodes are considered gaming classics.Excellent graphics,great gameplay and bits of mystery with just a touch of the surreal to not alienate those who like more conventional story arcs.Not just shooting enemies,but physics puzzles as well.If you like linear storytelling,the Half Life Series is the way to go.But be warned-the story doesn't have an ending as Valve has been dicking around with Episode 3(or Half Life 3 as it may be) so the series ends on a cliffhanger.When the next installment is,Valve isn't saying.
Go forth and have fun!