For free open source software, hardware recommendations,
instructions, plans and user manuals see HiveTool.org
The RAINEW 111 Wired Rain Gauge has an 8" diameter collector which meets NWS specifications for statistical accuracy. It can be purchased from Rainman Weather for $65.66.
The gauge provides a dry contact (a reed switch tripped by a magnet on the bucket). This can be wired to a digital input, such as one of the GPIO pins on the Pi (untested) or a device like the Phidget Interface Kit which provides 2 digital inputs, 2 analog inputs, and 2 digital outputs. The Phidget software has been modified to write the number of transitions to a file so the hivetool scripts can pick it up, log it graph it.