Free to play
The Trope Blender
Drop in two or three tropes and hit blend. Out comes a premise mashup you didn't know you were craving.
Free account, ten seconds — keeps this and everything else you make.
Pick two or three tropes and the Blender fuses them into one story idea with a built-in complication. The best mashups are the ones where the tropes rub against each other a little. Free, instant, gloriously specific.