Olympia Snowe will not run for re-election. This becomes a huge pick-up opportunity for Democrats and makes it measurably harder for the GOP to take control of the Senate. At this point, Democrats can reasonably expect to pick up seats in Maine and Massachusetts. They will probably lose seats in Nebraska and North Dakota (both open seats).
The makes Virginia, Wisconsin and New Mexico important open seats to hold along with Tester's Montana seat. They could conceivably pick up a seat in Nevada. Of those five races, they need only win three to maintain control of the Senate.
Of course, "control" of the Senate is worth a poop sandwich, since you really need 60 votes to do anything.
UPDATE: Interesting idea from Chait: Is Snowe prepping for a run from America Selects?