This poll shows that Angus King is overwhelmingly ahead in the U.S. Senate race in Maine this year. King is an independent candidate. No independent candidate for U.S. Senate from Maine has ever before received as much as 10% of the vote. Assuming King is elected, and assuming Bernie Sanders is re-elected in Vermont as an independent this year, that will make five instances in which someone who was not the Democratic or Republican nominee has won a U.S. Senate race in the period 2006 through 2012.
The other three were Lisa Murkowski in 2010, and Sanders and Joseph Lieberman in 2006. Thanks to Political Wire for the link.