well, the afsocket one. that's a completely nonblocking destination that implements udp/tcp/unix-stream/unix-dgram/syslog/network sources and destinations.
It basically boils down to using a LogWriter class that does the heavy lifting at least as long as your transport protocol is simple enough. Probably the simplest is to create a LogProto implementation and let the afsocket driver do the rest.
Asynchronous sources/destinations operate on a set of worker threads (in contrast to a dedicated thread) and are driven by the epoll() based event loop.