Okay, as far as the mainstream stories of Marvel... it's sadly mostly Avengerific. I suggest these for a quick catcheruper (in this order):
Civil War 1-7
Mighty Avengers 1-6
Fallen Son: The Death of Captain America 1-5
New Avengers 27-39
Thunderbolts 110-122 (Warren Ellis's Run)
New Avengers: Illuminati 1 and 5 (skip the middle)
New Avengers 40
Secret Invasion 1-8
Dark Avengers 1-current
New Avengers 48-current
Siege 1-2 (current)
And that puts you up to date. Now, that's the center of the Marvel Universe. To branch out from there, it's up to your personal tastes.
I suggest anything cosmic starting at Annihilation and on from there (continuing into Nova, then Annihilation Conquest, then Guardians of the Galaxy, and on).