Dark Horse released the first issue of their four-issue Mass Effect: Redemption comic book on iTunes today. The story occurs between the first Mass Effect game and the second one, which came out last month.
And if you download it today, it's absolutely free. So if you're interested in checking it out, act quickly -- this link will take you directly to iTunes.
I myself have not played Mass Effect, as I don't have an Xbox 360, but I have been very addicted to another BioWare-created game, Dragon Age, on the Playstation 3. If it's half as much fun shooting up aliens in Mass Effect as it is fighting Darkspawn in DA, then it's probably worth checking out as well.