The first is that BioWare has decided to add to the game's ending via free DLC this summer. Entitled "Extended Cut," the DLC will add additional cinematic sequences and epilogue scenes to Mass Effect 3, and you can read the official press release here and a mini-FAQ here for all the details.
Next up, the "Resurgence Pack," a Multiplayer DLC add-on, will be released on April 10th for free, and will include several new maps, character classes, and weapons. You can check out the trailer here or below and read the official blog post which details all the content right here.
Lastly, a patch for the game will be released on April 9th for PC owners and April 10th for console owners. Full details on what issues the patch will address can be found here.