It consists of a client and a server. The client is usually embedded into your favorite emulator and the server is a stand-alone application that needs to be run on a machine directly wired to the Internet.

Kaillera features:

• Small, fast and efficient C++ code.
• UDP use for better latency.
• Intelligent networking cache code.
• Multi-platform support.
• Low to no lag for players with good ping.
• Works through firewalls.
• LANs/WANs support.

• various versions of MAME
• MameLang32+ (MAME32 with lots of features added)
• Bliss (Intellivision emulator)
• NESten (NES emulator)
• Jnes (NES emulator)
• Nemu64 (Nintendo 64 emulator)
• Modeler (Sega System 32 emulator)
• Gens (Sega Megadrive emulator)
• WinUAE (Amiga emulator)

License: free