There are several, actually. Trapster is the one that's been around the longest. It has a lot of interactivity, with other users providing near real-time updates to radar trap locations.
For those who would rather not use their phones for this purpose, some of the high-end radar detectors are able to link together using a similar network.
I thought apps like trapster were taken offline because police objected.
Also relying on other people is not as reliable as using the police’s own vehicle lojacks.