Serasidis have developed simple and effective AVR oscilloscope circuit. Recently he updated its software to V2.0 which ads some great features like ability to see on LCD Volts peak to peak, Volts DC and frequency of measured signal.
Also there are some hidden improvements like significant RAM usage reduction from 2kB to 335 bytes. Improved sampling rate and few bug fixes. Of you have built one for yourself, upgrade firmware and enjoy new features.