The Sea Wasp (Ryk E. Spoor)

You can play any of the ME games without playing the others; ME2 has an introductory "motion comic"/"choose your own adventure" segment for those who don't import a gamesave from ME1. (ME3 has a set of default choices if you don't import a gamesave from ME2... but most folks won't like the story the results from those defaults.)

That being said, I personally would recommend playing the games in sequence. You get more investment in "your" Shepard that way, and the story reveals will have more impact if you've managed to make it this far without them being spoiled.

Be advised that the mechanics of ME1 are a bit primitive; if you don't "level" a gun skill then that type of gun will be very difficult to use, and there's no "VATS" like mechanic like in the new Fallout titles. (Easily avoided for by taking one of the "power"-heavy classes like Adept or Engineer, though, as powers do have a pause-game-to-use feature.) ME2 and ME3 have better mechanics and actually make for good shooter games (if you want to play them as such) as well as solid RPGs.

-- Steve's still mildly addicted to ME3's multiplayer, which is superior to that of many dedicated shooter games he's tried.

PS: the added downloadable content for ME1 is easily skipped; none of it gets reflected in later games, save for a few non-interactive mentions. ME2 has four DLC missions that I'd strongly recommend getting; Overlord (which sets up a mission in ME3, plus it's amazingly creepy in an eldrich-horror sense), Stolen Memory (introduces a secondary character in ME3), Lair of the Shadow Broker (huge, huge set-up for ME3 and it may be the DLC mission I've enjoyed most from any game), and Arrival (bridges ME2 to ME3).