There are quite a number of these out there, but this was a new one for me - one of the better ones I’ve seen. http://www.airnav.com/AIRNAV is great for planning flights that include consideration of fuel prices; also other good info, like fuel prices in the vicinity a given airport, FBO info, etc.
A quick registration process is needed if you want to be able to update prices (prices are quite actively maintained by users).
- Mike.