In Senate races, I've only found six that have announced challengers, so far. Three of these are open seats, and then the three most vulnerable Republican seats. Anyone heard anything else? I don't expect a lot of candidates to climb out for a while, but let's speculate, here.
Maryland (D) D: Kweise Mfume, Ben Cardin.
R: Michael Steele.
Minnesota (D)D: Amy Klobuchar, Patty Wetterling
R: Mark Kennedy
Montana (R)R: Sen. Conrad Burns (inc)
D: John Morrison
Pennsylvania (R)R: Sen. Rick Santorum (inc)
D: Bob Casey, Chuck Pennacchio
Rhode Island (R)R: Sen. Lincoln Chafee (inc)
D: Sheldon Whitehouse, Matt Brown
Tennessee (R)R: Ed Bryant, Bob Corker
D: Harold Ford, Jr.
Politics1.com has a good updated list of active candidates.
Connecticut (D)D: Sen. Joe Lieberman (inc), John Orman.
Delaware (D)D: Sen. Tom Carper (inc)
R: Colin Bonini
Michigan (D)D: Sen. Debbie Stabenow (inc)
R: Bart Baron, Nasser Beydoun, Keith Butler.
Mississippi (R)R: Sen. Trent Lott (inc)
D: Erik Fleming.
New Mexico (D)D: Sen. Jeff Bingaman (inc)
R: Tom Benavides
New York (D)D: Sen. Hillary Clinton (inc)
R: Adam Brecht
Utah (R)R: Sen. Orrin Hatch (inc)
D: Pete Ashdown
Vermont (I)I: Bernie Sanders
R: Greg Parke
West Virginia (D)D: Sen. Robert Byrd (inc)
R: Hiram Lewis
Wisconsin (D)D: Sen. Herb Kohl (inc)
R: Robert Gerald Lorge