This is a neat little service that simplifies getting geocache data onto your smartphone (from a well-known geocaching website). Rather than manually entering the coordinates for each cache, you can just hit a button - the coordinates and description are sent to your phone.
It consists of a Greasemonkey script (currently only tested in Chrome), and a PHP script (which should be popped on a handy server). The Greasemonkey script adds a couple of buttons to cache pages. These allow you to queue up caches, and then send this queue off to the PHP script. Your queue then gets bundled up into a .gpx file and emailed to you. From there, you can open it in your GPS app of choice.
Why use this instead of the official app? Other GPS apps offer some nice conveniences like downloadable maps and track recording.