RakNet has been aquired by OculusVR and is now on github under an open source license:
The low-level feature set is similar to ENET (guaranteed messaging over UDP), but it also has a number of higher level features (voice chat, object replication, RPC, etc...). We're using RakNet for 5 years now and it is a solid and low-overhead messaging library, it's also used in many other game projects, and as far as I know it is also the networking layer in the Unity engine.
The most exciting thing to me is that open-sourcing the lib could enable an emscripten port (there is already a Native Client port which probably could be used as a starting point).
It's probably also a good idea to create a "RakNet lite" which doesn't have all the high-level bells'n'whistles (connection management and messaging are the really important features imho).
Cheers,
-Floh.