Aaron Blake reports that Kansas Republican Senator Sam Brownback will not be running for reelection in 2010: Not in 2008 anymore: 2010 contest for Kansas Senate heats up in a hurry..
Brownback has a extraordinary record on trade. Cato indicates that, since 1997, he's voted against trade barriers 91% of the time. Not only that, he's opposed trade subsidies 69% of the time.
Chris Cillizza lists the candidates lining up to take his place (Friday Senate Line: The 2010 Landscape):
Cillizza provides overviews of nine other 2010 Senate races. Wikipedia has also set up its 2010 Senate races page: United States Senate races: 2010.