DyConnect

A networking library written in C++. The main features are:

multi-threaded TCP/IP programming
UDP socket wrapper
IPv4 and IPv6 support
compression/decompression routines implemented with zlib
CRC hashing using the 'fast' CRC algorithm
cross platform (Windows and Linux)
provides non-blocking network functions
distributed under the zlib license, meaning it's free and you're under no obligation to release your source code
allows general purpose networking. There are no hidden or custom protocols, and you can use DyConnect to communicate with non-DyConnect applications.

DyConnect includes the high-level Dasyne classes, used for rapid development of networked games. Dasyne features:

built on top of DyConnect
allows you to create variables that are automatically synchronized between the server and clients
detects and corrects any synchronization errors
only the parts of the data that have changed are sent to clients
can synchronize very large amounts of data, limited only by the rate of change
completely non-blocking
easily switches between TCP and UDP
all network messages are compressed