The Open AD&D Toolkit is a free, open source C++ API for Linux RPG development. It is in an early stage of development. It is based upon the Advanced Dungeons & Dragons (R) 2nd Edition rules. The core API will be written in pure, standard C++ and will be released under the GNU General Public License.
The Toolkit is meant to be independent of user interface. SVGALib, MUD-like text interfaces, rogue-like text interfaces, X11 interfaces and Crystal Space are all possibilities.
Although the Toolkit is designed for and written on Linux, ports to other Unix-like OSes should be straightforward or trivial.