I bought one of these Logic Analyzers from Saleae - #1 with Professional Engineers about a year ago. It works great and has saved me many hours while trouble-shooting. Not only does it display logic levels but it has built-in code to display the contents of SPI, I2C, etc messages! At $150 it's a steal. (and the hardware fits in a shirt pocket with room left for your smartphone)
For the record I'm in no way affiliated with the manufacturer.