Aerosoft has a nice one. It's way more than just a fuel calculator though. Sort of an electronic E6. You can do Time/Speed/Distance, Wind, Altitude and Fuel. If you have FSUIPC it can connect with the sim and get a lot of variables automatically.

Only problem I had was with the download. Seems the versions (FSUIPC Registered) and (FSUIPC Unregistered) have gotten switched on the download page. I payed for the Registered version and got the Unregistered but a post in the Aerosoft forum got that straightened out quickly. Been using it a lot lately and it works great.

Only major bummer is that it can't be run from within Fs9 unless you are running Fs9 in Window Mode.

http://aerosoft-shop.com/product_info.php?products_id=59