I got a Gabotronics XProtolab GT-0010, which comes with a 128x96 black-and-white OLED and scope/FFT firmware. It's smaller than the Cascata device I built out of an Arduino and Sparkfun Nokia LCD board, but it's black and white instead of color, and the display is about 1/2 the size.
Sadly, the scope app isn't open source, but the framework is. You need an Atmel ICP to program it; the onboard USB doesn't do that (or much of anything).
FFT:
Scope:
