If you own a Saleae Logic analyizer you may be interested in PWM Logic.
It is free for non-commercial use.
From **broken link removed**
Features
8 channel PWM generator.
Frequency range from 1 Hz to 1 MHz.
Output sample rate up to 4 MHz
Both synchronous and asynchronous PWM signal generation.
Easy to use User interface.
R/C model control preset.
Saving and loading of PWM generator settings.