Ambac

Ambac Financial Group used to give an A+ to school bonds, until its own ratings fell to C's. Ambac Assurance, the holding company's primary subsidiary, sold financial guarantee insurance and other credit enhancement products for municipal bonds in the US market. However, the company has halted all new business and has placed its remaining business in "run-off" -- meaning it is only taking in premium payments and paying out claims as it is able. Already operating under "rehabilitation" by Wisconsin regulators, Ambac Financial filed for Chapter 11 bankruptcy in 2010.