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):
DELTA (DEscriptive Language for TAxonomy)
MEKA (Multi-Entry Key Algorithm)


Other Web-based Keys:

 ACTKEY (including an interactive key to the Angiosperms)
(NOTE: Link no longer current, but try this link)


FLORIGATOR
A multiple entry key for flowering plant family identification