There are 34 Senate seats up for re-election in 2008 (33 Class II seats plus a second unfinished term in Wyoming).

• Alabama
• Alaska
• Arkansas
• Colorado
  
• Delaware
• Georgia
• Idaho
• Illinois
• Iowa
• Kansas
  
• Kentucky
• Louisiana
• Maine
• Massachusetts
• Michigan
  
• Minnesota
• Mississippi
  
• Montana
• Nebraska
  
• New Hampshire
• New Jersey
• New Mexico
• North Carolina
• Oklahoma
  
• Oregon
• Rhode Island
• South Carolina
• South Dakota
• Tennessee
• Texas
• Virginia
• West Virginia
• Wyoming
• Wyoming (thru 2012)

This Wikipedia page is a good resource.