by praveen @ circuitstoday.com:
Many guys here were asking for a frequency counter and at last I got enough time to make one. This frequency counter using arduino is based on the UNO version and can count up to 40KHz. A 16×2 LCD display is used for displaying the frequency count. The circuit has minimum external components and directly counts the frequency. Any way the amplitude of the input frequency must not be greater than 5V. If you want to measure signals over than 5V, additional limiting circuits have to be added and i will show it some other time. Now just do it with 5V signals.
Frequency counter using arduino – [Link]