UK Speed Cameras - This mashup plots locations of speed cameras for the motorists in the UK looking to avoid the fuzz (and ensuing speeding fines)
BlogMapping - This search tool allows you to find locations of bloggers in the U.S. by zip code. Results are, of course, plotted on a Google Map.
The World of Kingdom of Loathing - I'm not entirely sure what "The Kingdom of Loathing" *is*, but this mashup has some really cool features to it. First, latitude and longitude is constantly displayed as you roll the mouse along, and second, an additional map shows up in the push-pin pop-ups giving you another zoomed in view to work with.
Attention Bob Dylan fans - Here is a Google Maps plot of where Bob Dylan has been and where he's going (sounds like one of his songs). It hooks in with setlists and lots of info about his current North American tour.
And finally, here is a Google Earth hack that gives you the latest news, links and info for English Premiership Football Clubs. It links to the BBC Local areas for a club, the location of the grounds and route planning between them. Check here for the instructions to enable this data for your Google Earth use.