Bioware loves to pack all sorts of stuff into DLC, for better or for worse, but the Lair Of The Shadow Broker quest for Mass Effect 2 is an example of the good side of the equation. It's by far the biggest DLC the company has done, and it not only gives you more background on Liara but starts to lay down plot threads for Mass Effect 3. You can play this at any time, but it's recommended you take it on after the game's climactic suicide mission for the best results.