The TF2Maps.net Feedback system is a one of a kind system designed to give players a way to easily and effectively give feedback to level designers while still playing the game, as opposed to having to do a laborious write up after a playthrough. From launch the system has been a massive success and has assisted level designers in making some truly amazing levels - last I check it had been used in the creation of over 4,500 maps.
Originally built in a spat of rapid iteration of the course of two weeks, the code base is hardly the most beautiful piece of software engineering to have been written (rather embarrsingly one of the earlier versions caused major down time when the error reporting code got stuck in an infinite loop and filled the site's hard drive) however at the time of writing it has survived the test of time and various attempts to rewrite a similar system from the ground up.
On Christmas Eve, 2010, Valve awarded me a Community weapon for Team Fortress 2 in respect of my work on the TF2 Feedback system and my contributions to TF2 Prophunt. In 2012, the staff of TF2Maps.net followed up by awarding me with the "Map Maker's Medallion" for "notable and significant contributions to the TF2 Mapping community."
In 2014, Valve used feedback gathered from the system to improve their maps in public beta at the time: Cactus Canyon and Asteroid. The system was also used to gather feedback on many community maps that are now officially included with the game.