GPS receiver
From GeoWiki
To go geocaching, you'll need a GPS receiver. This receives satellite signals and pinpoints your current position. By entering coordinates of the Geocache, your GPS will typically display an arrow giving you the distance and bearing of the Geocache.
The most popular GPS brands are Garmin and Magellan. Prices vary dramatically, so it's best to ask in the forum to see where to get one at the moment.
Every now and again there are polls in the forum about the popularity of individual brands. Here's the most recent poll.