I'll admit I've been pretty jaded when it comes to mashups. I mean, besides the revolution that
Yahoo! Pipes is finally making in the hackneyed "Web 2.0" era, the word has become synonymous with just about any web page that uses Google Maps. Yaaawwwwn.
Then, while looking for some runs to do while in San Francisco for my upcoming honeymoon this June, I came across
WalkJogRun.net. At first glance this looks like just about every other mashup out there:

However, there's more going on than meets the eye - it's more than just putting pushpin on a map - you can actually map out entire routes and get mileage, average time and speed, hill difficulty and approximate calorie burn for your weight:
As you plot your run, WJR adds mileage markers indicating just how far you'd be going and where in the route you'd be hitting your splits. This is a bit of the Holy Grail for runners: For some time, the options for measuring out a route have been limited - you could use a pedometer or GPS, but this usually entails having to walk, jog or run the route before hand. With WJR, you can find or plot a route before even thinking about lacing up.
Of course there's also the "aggregation" effect that a web app like WJR offers - it's pretty cool to check out some other runner's favourite routes from near or far. I can get lost for hours just poking around the maps.
My only criticisms are that the site is a bit slow, incomplete feature-wise, a little lacking on the social networking aspect and absent of features for uploading pictures or routes from GPS units like the Garmin 205/305. I'd also like the ability to see how long a run would take by pace (ie mins/mi or mins/km) instead of raw ground speed.
Now, all that said there are 51,865 routes in the database from all around the world and on this and sheer cool factor alone, it's worth checking out.