Kathy Sierra writes about "out-of-context user delights" on the Creating Passionate Users blog. The idea is to enhance the user's experience with your product by surprising them with something out of context. She gives numerous examples; here is just one of them:
Context: Company that creates Business PresentationsNot only does the "out-of-context user delight" increase the user's satisfaction, it also makes them more likely to help you market via word of mouth (to an extent greater than you might ordinarily expect from a satisfied user).
Delightful Out-of-Context Surprise: Their website includes a main menu choice for "Staff Tattoos"