Jon at the codewrite website has posted a neat project: Arduino Uno Capacitance Meter. The project is very simple as it uses one capacitor and about 40 lines of code. The meter function can measure capacitance from 1 pF to 1000 uF with 1 to 5% accuracy from 3.5 pF to 1200 pf. Jon has improved the design with the Capacitance Meter Mk II.