Nokia has been the largest Windows Phone hardware partner thus far. This is why Microsoft acquired Nokia's devices and service businesses this past September for $7.2 billion.
Microsoft is also negotiating with Chinese ZTE Corporation, in order to allow ZTE to build low-end Windows Phone smartphones in the U.S., Europe and China.
Sony has partnered with Microsoft on building a prototype of a Windows Phone 7 powered smartphone a few years ago. The Japanese company also developed the Sony Ericsson Xperia X1 and Sony Ericsson Xperia X2 in 2009. Both were running on Microsoft's Windows Mobile OS.