For Mass Effect 1, Bring Down the Sky is free on PC as mentioned. Pinnacle Station is cool but pretty skippable, in my opinion. According to the wiki, it's included in the PC version of the Mass Effect trilogy if you happen to have that.
For Mass Effect 2:
Lair of the Shadow Broker - Arguably the best DLC of the entire trilogy along with Mass Effect 3's Citadel. Definitely worth it, and lots of good story with Liara and the Shadow Broker.
Arrival - A pretty good DLC that leads things into Mass Effect 3. It's the most connected to the main story arc of the Reapers of the ME2 DLC.
Overlord - In my opinion it's the worst of the story DLC, but it's still not bad. They tried to push the Hammerhead vehicle in this one, so you'll be using it a fair bit. When importing a save, it has some characters you'll meet again briefly in Mass Effect 3.
Stolen Memory - The story mission itself is pretty short, but it brings in a character that I obviously think is great =P
The rest are all appearance and weapon packs that can very easily be skipped without missing anything. Cerberus Network does have The Price of Revenge with Zaeed which is on par with Stolen Memory, but you should get it for free.