Dave's own version of Citadel (DOC) is a variant of the Citadel/UX Bulletin board system (BBS) software which was developed specifically to run ISCA BBS in the late 1980s. It is based on Citadel/UX 3.0 but very heavily modified to suit the specific needs of ISCA BBS, which at its peak was a massive system supporting hundreds of simultaneous users. DOC and its descendants are now used by a number of Internet BBSes, including some in Latin America, which have been heavily modified, support 8-bit characters, and are presented in Spanish.
Along with its offshoots DOC has inspired at least four clone codebases, including YAWC, which is likely the oldest, Jammin (code), which has since become WeIrDo, bbs100 (which is licensed under GPL), and A better Citadel (ABC).
After a number of years, the DOC source code was finally released under the GNU General Public License. It is presented mainly for 'academic interest' due to its complexity, specificity to ISCA BBSes PA-RISC hardware, and poor documentation; however, at least one branch from an earlier version of DOC is being developed under the name vDOC (or variant DOC), which is known to work on Linux, Solaris, and FreeBSD systems.
Partial list of DOC BBSes
Unless otherwise noted, all these BBSes may be accessed by telnet.