hey there,
We are using eAthena.
eAthena is coded in the C programming language and makes use of the zlib and MySQL libraries. The actual application is split into three servers - login, char, and map, each of which serve a different purpose. Support for multiple map servers is also possible. Thus, while the individual eAthena servers are single-threaded, they do essentially have support for running on multiple processes, and even on multiple computers.
For storage of dynamic game data, eAthena can be compiled to use one of two different modes of database operation: TXT or SQL. TXT is short for plain-text whereas SQL is MySQL. SQL is the standard way of running the eAthena servers. TXT is generally used merely only for testing and development purposes as a quick and convenient way to get eAthena up and running without having to deal with any 3rd-party software, and is not recommended for live, production servers.
eAthena uses plain-text databases for configuration of static game data such as item, monster, and skill information, although some of these can optionally be configured to use MySQL instead. Additionally, eAthena features a fully custom, C-like scripting engine for configuration of NPCs, quests, warp portals, and monster spawns, among other interactive in-game systems.
There is more information about the server on the servers wiki.
Regards,