Ah, 'tis awesome to read something completely positive about Mass Effect again.
If your looking into some of the additional media stuff, there's also a few books, comics and an iOS/Android game, all of which end up getting referenced in ME2/3.
The most directly impacting on the in game events would be the comic Redemption, set between ME1&2, dealing with what Liara got up to between games. This you should probably read after you meet up with Liara again in 2 though, as it does have some big spoilers for that meeting and the start of the game.
Infact, while we're on the subject of Liara, in ME2, because its the PS3 version, the DLC Lair of the Shadowbroker, which heavily involves her, is included on disc, and can be done at any time. However, its meant to be done after you've finish the main story, which can potentially give you a lot of additional dialogue and awesomeness, depending on what you did leading up to the end. Assuming you're alive of course...
Novel wise you could get away with just reading Ascension, which isn't vital to anything, but explains a few characters annimosities, and you meet some of the people involved. It also gives some excellent insight into Quarian society and culture.
The book set before it, Revelation, is set before ME1, involves Saren and Anderson, and introduces Khalee Sanders, the main character in the books, who is also in ME3.
The book set after, Retribution, spoils events for the end of ME2, and introduces a few important characters and plot points for 3. It is, however, the point where the wheels started to come off the franchise, retconing a few events and removing one previously important choice from ME1. It also ironically ends up getting retconned itself more than once. It's still a genuinely good read in and of itself though.
Do not, under any circumstances, read the novel Deception though. It is amazingly bad, and written by someone who apparently had no knowledge whatsoever of the franchise. It was so awful there was an official apology for it, and its supposedly going to be being rewritten to remove the dozens and dozens of mistakes it makes. But even if those weren't there, its still a crap story.
On the other hand, the App Game Mass Effect Galaxy is a fun little top down shooter with its own mini dialogue wheel, choices and branching story path, involving two squadmates from ME2, and a few NPC's you meet along the way.
Other than Revelation, you'd be better off at least starting ME2 before getting into any of this other stuff though.
Edited by Total Biscuit, 05 January 2013 - 03:54 AM.
No, I'm not 'that' TotalBiscuit.