Asuswrt-Merlin is a third party firmware for select Asus wireless routers. Based on the Asuswrt firmware developed by Asus, it brings tweaks, new features and other improvements to the original firmware, while retaining its performance and ease of use.
Note that only downloads are hosted on SF.net - the complete source code can be found on https://github.com/RMerl/asuswrt-merlin (legacy 380.xx) and https://github.com/RMerl/asuswrt-merlin.ng (382 and newer releases).