Lair of the Shadow Broker
I completed Mass Effect 2 a few months ago, and although I was glad to get the main quest finished I was still craving some more of it's in depth storyline, and who can resist finally unveiling the puppet master of both games, the shadow broker.
Available on live for 800MS points, it's not too expensive but I really wish I'd picked it up in one of the sales. I finished it in around 5 hours, and without giving anything away it was at least a little challenging with a couple of boss fights and some great set-pieces. I quite enjoyed the "chase" section before the first boss fight, but I really could have done without the Star War episode 2 car chase. Mass effect vehicle mini games have never managed to do anything but annoy me.
It's hard to give a comprehensive review without major spoilers, but I will say it's well worth playing and I am intrigued to see how it will effect Mass Effect 3. I would also advise that you play this as part of the main storyline as some of the elements will defiantly help the main quest.