RakNet's (<a href="http://www.jenkinssoftware.com/" rel="nofollow">http://www.jenkinssoftware.com/</a>) 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 (<a href="http://enet.bespin.org/Features.html" rel="nofollow">http://enet.bespin.org/Features.html</a>) has a similar low-level feature set, but lacks RakNet's high-level features as far as I know (replication RPC, voice etc).