I started playing with a Fotek SSR-25DD solid state relay that I accidentally purchased off Amazon quite a few months ago. This SSR is made in China and very affordable (in the ~$10 range). The specs are not very clear.
The question of the day is: how useful is this for PWM with a DC load. A simple test jig with a signal generator yields the ceiling. At 50% duty cycle, this SSR stops being able to switch at about 320 hertz.
The next question: what is the max frequency you can get full range of duty cycles.