RakNet's (http://www.jenkinssoftware.com/) low-level killer feature is a tweakable guaranteed-messaging-layer on top of UDP. One can send messages as unreliable-unordered, reliable-unordered, unreliable-ordered and reliable-ordered. Messages can be sent in ordering channels (messages in one channel are only ordered between each other, not with messages in other channels). ENET (http://enet.bespin.org/Features.html) has a similar low-level feature set, but lacks RakNet's high-level features as far as I know (replication RPC, voice etc).
Yeah I recently discovered enet through Emscripten (which emulates UDP on top of SCTP. SCTP is the protocol used by WebRTC and supports reliable and unreliable packets like enet and RakNet, though I believe Emscripten's UDP emulation only uses it in unreliable mode (since UDP is unreliable).
Apparently SCTP can be tunnelled over UDP if native support is unavailable (which seems very likely), which means I've ended up with the crazy stack of:
enet, which uses UDP which is emulated using SCTP which is tunnelled through UDP.
Crazy but it works. Bring on the real time web games!
There are some interesting issues already appearing on GitHub for this, buffer overflows and SQL injections. Hopefully open sourcing this should make it more secure as well as free to use.
I've been looking at networking libraries recently so this news has come at a good time.
