The dictionaries that appear on Dictionary.com include:

The American Heritage® Dictionary of the English Language, Fourth Edition

Webster's Revised Unabridged Dictionary

WordNet (Princeton University)

The Free On-line Dictionary of Computing

Jargon File

CIA World Factbook

Easton's 1897 Bible Dictionary

Hitchcock's Bible Names Dictionary

U.S. Gazetteer

Through this site you can access definitions from:

Acronym Finder

On-line Medical Dictionary