The Displace filter is probably a filter that is used very little by most Photoshop users However, it is a very useful filter for achieving various effects. Here are links to two sites that provide some great tutorials for the use of the Displace filter and displacement maps.
The Golden Mean The Golden mean has 8 pages of tutorials and examples of how you can use the displace filter.
Technical Slop: Technical Slop hasn’t been updated for quite sometime now. However it has some great tutorials and examples that will help you understand the and master the Displace filter.