I like using the online dictionary at die.net because it’s fast and clean.
It’s easy to query, also. Just append your word to the end of the URL. For example:
It describes itself as offering “free cross-referenced definitions, spelling correction, and word searches from WordNet, Webster’s, FOLDOC, and a variety of specialized sources.”
In the “spam” entry above, some of the sources include the Free On-Line Dictionary of Computing, the Virtual Entity of Relevant Acronyms, and the Jargon File.