Identification Web Links:

FAMILY IDENTIFICATION: Flash Cards
(contributed by Steve O'Kane, University of Northern Iowa)

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 three (listed in the link above as well):
DELTA (DEscriptive Language for TAxonomy)
LUCID (Lucidcentral Identification and Diagnostic Tools)
MEKA (Multi-Entry Key Algorithm)

Other Web-based Keys:
 ACTKEY 
HERE is a still active link to a Key to the Angiosperms
FLORIGATOR A multiple entry key for flowering plant family identification

Lucid Key to Amazonian Ferns and Neotropical Triplophyllum