Compared to the near-contemporaneous Dragon Age and to its own sequels, Mass Effect has surprisingly little downloadable content. Mind you, I actually consider that as a blessing. One of the DLCs is story-based, featuring a sidequest on an asteroid, while the other one is more recreational, adding a space station housing a combat simulator. They also come with some juicy rewards and make reaching level 60 that much easier, should that be your goal.
Bring Down the Sky: 
Shepard receives a message informing her/him that a gang of batarian terrorists have hijacked an asteroid and are planning to crash it into Terra Firma, a human colony world. The goal is simple: stop them at all costs. This is a lengthy sidequest, about the same length as a storyline mission in the main game. It notably serves to officially introduce the batarian species, humanoids with leathery patterned skin, four eyes, fleshy moustache-like ridges on their face and teeth that look like sharpened whale baleens, who live under an autocratic regime that allows slavery. The sidequest also features a final decision that may give you pause. Moreover, certain parts can be challenging: there are lots of turrets involved, including mobile ones, and one brief, but annoying sequence where you need to navigate a minefield. On the plus side, you can pick up some very nice loot, including biotic amps, omni-tools and even some quarian armour, all of which are otherwise fairly rare. All in all, a worthwhile addition to the game.
Pinnacle Station: 
This one I’m distinctly less enthusiastic about, as I’ve never much enjoyed combat arenas. Shepard receives an invitation to Pinnacle Station, a space station that houses a combat simulator, run by Admiral Ahern. The inbuilt safeties in the simulator mean that your team can’t actually die; they’ll simply fail the session if that happens. The session will also end if the timer runs out, and grenades and medigel will be replenished at the end of each bout. There are four combat modes, with two possible maps at first; when you beat the high score for both, a third, more difficult map becomes available in each mode. If you manage to beat the high score for all of those, Ahern will offer a special scenario, with his retirement pad as a reward. The catch is that the safeties will be turned off, so Shepard can die for this one. The apartment is worth it, though, because, aside from some redundant grenade and medigel dispensers, it contains a computer from which Shepard can order gear from Alliance warships. You can’t actually decide what you’ll get: Shepard simply pays the commander of the ship a set sum and gets a random item of a certain level in return. Obviously, the most expensive price tag means the best gear. This is the main perk of the DLC for me, but if you enjoy combat, then you’ll probably like it more than I did. Maybe not on Hardcore or Insanity, though.