DD-WRT is a Linux based alternative OpenSource firmware suitable for a great variety of WLAN routers and embedded systems. The main emphasis lies on providing the easiest possible handling while at the same time supporting a great number of functionalities within the framework of the respective hardware platform used.
Not sure if your router is support check out this page where you just type in your router model and it dynamically checks it for you. After you get a result, click on it and it will give you the forum and wiki for that specific router. The wiki is the install instructions for flashing your router.