How about this: http://www.picaxe.com/Hardware/Add-on-M ... PCB-scope/
Of course the specs are nothing to write home about. The supporting software is Windows-only.
I've found it perfectly OK for watching - and measuring - PWM pulses on an RPi. It also has a logic analyser (digital signal levels) mode.
Some simple soldering required to add the headers to the board.
The price makes it a good match for an RPi.
Alternatively, if you only need to watch digital signals on the RPi header, a logic analyser might be all you need. You can find usb-connected, windows-software supported logic analyser dongles for a similar price. For example: http://hobbycomponents.com/test/243-hob ... c-analyser
This takes 'snapshots', it doesn't run in real time.
No illusions: these are not workshop or lab quality measuring instruments. But they allow me to see and understand events that are not visible with an 'analogue' multimeter.
I own both. I have no connection with either company mentioned.