Asuswrt is a unified firmware developed by Asus for use in their recent routers. The firmware was originally based on Tomato-RT/Tomato-USB, but has since seen many changes. Asus started using this new firmware with their recent routers like the RT-N66U, but they also started moving other routers to this new firmware, like the DSL-N55 or the RT-N56.
Being mostly based on GPL code, almost all the source code and all necessary build tools is available from their website. There are a few proprietary components that are closed source (like the wireless drivers from Broadcom/Ralink). In these cases, Asus includes binary-only versions of these files. In the end, their GPL release includes everything needed to completely recompile a working firmware, with the exact same features as found in their firmware releases.
One big advantage of Asus going with a unified firmware is that they have one single code base for all their supported routers. That way, bugfixes for one device can automatically be applied to all other supported devices. Same when they start adding new features (such as AiCloud), it's easier for them to support multiple routers at the same time with these news features. And finally, it means that support for a router does not die the minute a newer router replaces it on the market. The RT-N16 still gets updated as frequently as the RT-N66U, for instance (as they share very similar hardware).