Identification Web Links:


Programs for Interactive Identification and Information Retrieval (by M. J. Dallwitz)
  A current and thorough listing of numerous programs;
also includes links to on-line articles by Dallwitz et al. on principles,
comparisons, and uses of interactive identification keys

Here are two (listed in the link above as well):

ACTKEY 
(including an interactive key to the Angiosperms)

 DELTA (DEscriptive Language for TAxonomy)

MEKA (Multi-Entry Key Algorithm)