Recently I've had some fun building tinymap.net which is best described as tinyurl for Google maps. You scribble on the map, and then you get a short URL to it. For example, to see my average trip to work go to http://www.tinymap.net/MkJVFx77KSX/.
I of course did no market research or planning and simply started coding, and have since found a few other similar (and fairly cool) sites such as Quikmaps and wayfaring. My site certainly has them beat in the KISS stakes, although it does involve a mashup, some AJAX and has been tagged a lot on del.icio.us so I'm claiming it is Web 2.0!