6 channel PWM with USB support
You don't mention other requirements, like Flash and RAM size, and other I/Os, so I'll have just to take a shot at it. For most parts Digikey has very good selection tools, but microcontrollers are a bit of a let-down: you can't select for USB separately, for instance, only combined with other features, which makes the selection process rather involved. Also no selection on PWMs.

But like jippie says you can find information on manufacturers' sites. On the Atmel parametric search page I found for instance the AT32UC3A:


Full Speed USB + USB-on-the-go  
7 PWM channels  
128 KB to 512 KB Flash  
32 KB or 64 KB SRAM  
100 or 144 pin QFP, 144 pin BGA  


If this is not the device you're looking for, then please update the question, or have a look at the parametric search page I linked to.