Carl Levin's packing it in. I was a teenager when he first got elected to the Senate in 1978; he's opting not to run for reelection when his term expires in early 2015. At age 78, he's old enough where you start to wonder whether another term is a wise idea, although he doesn't seem to be slowing down much mentally.
He's been a hard candidate to beat, as he puts forward the persona of an avuncular Pentagon budget-trimmer, death to the $5000 toliet seat (and to more worthy weapons systems as well). That persona has left Republicans to say he's more liberal than Michigan is, but that hasn't worked yet.
It's a bit too early to pick successors, but state AG Bill Schuette might well be an early favorite for the GOP nomination. Had Schuette not ran against Levin in 1990, he would likely be in the spot Dave Camp is now; Schuette had Camp's House seat until then and Camp didn't give it back, riding the safe Midland-area seat into a Ways and Means charmanship.
Any number of other folks like Reps. Mike Rodgers and Candiace Miller, former Rep Pete Hoekstra and former AG Mike Cox would be the other names that would be on the short list for Senate on the Republican side. Hoeskta and Cox lost a hard-fought gubernatorial primary in 2010 and Hoesktra was the 2012 nominee for Senate, getting hosed by Debbie Stabenow.
The Democrats have some interesting possibilities, especially when there is a governor's race in 2014 as well. Former Gov. Granholm might be an interesting choice, as might 2010 governor nominee Virg Bernero if he doesn't want to have a rematch with Gov Snyder.
Next year should be interesting; an open Senate seat and a hotly-contested governor's race with some union bile flowing after Snyder snuck a right-to-work (a.k.a. don't have to pay union dues if you don't want to) bill through the lame-duck legistature in December.