Other Puzzlehunts
Microsoft College Puzzle Challenge   http://www.collegepuzzlechallenge.com/
Melbourne University Puzzle Hunt http://www.ms.unimelb.edu.au/~mums/puzzlehunt/
The Game (Bay Area)   http://chappie.stanford.edu/~snitch/game/game.html
Miami Herald Tropic Hunt   http://www.vwtech.com/tropichunt/
Other Lists of Puzzle Sites  
Crossword links (Ray Hamel) http://www.primate.wisc.edu/people/hamel/cp.html
Puzzle links (Google directory) http://directory.google.com/Top/Games/Puzzles/
Organizations and Conventions
American Crossword Puzzle Tournament — annual crossword tournament and de facto puzzle convention in Stamford, CT http://www.crosswordtournament.com/
National Puzzlers' League — puzzle society whose members include many Mystery Hunters as well as professional puzzle constructors, holds an annual convention in July http://www.puzzlers.org/dokuwiki/doku.php
Solving/Constructing Aids
The Internet Anagram Server — anagram solver/generator http://www.wordsmith.org/anagram/index.html
Purdue dictionaries and wordlists — lots of lists in lots of languages (via ftp) ftp://coast.cs.purdue.edu/pub/dict/
National Puzzlers' League members area — search or download word lists (paid membership required) http://www.puzzlers.org/dokuwiki/doku.php?id=secure
Puzzles Online
Atlantic Monthly — monthly cryptic crossword http://www.theatlantic.com/doc/index/the_puzzler
mathpuzzle.com — Math, logic and other puzzles, updated weekly http://www.mathpuzzle.com/
New York Times — daily crossword (subscription required) http://www.nytimes.com/
Puzzability — company providing puzzles on their web site, and custom-order puzzles http://www.puzzability.com/index.shtml
Syndicate — Vocabulary Word Puzzles (and a biweekly crossword) http://syndicate.com/
Conceptis puzzle — PBNs and similar puzzles (free weekly puzzles with registration) http://www.conceptispuzzles.com/
Games Magazine's site — Crosswords, PbNs, and some other content from the print magazine. http://www.gamesmagazine-online.com/
Triple Play Puzzles — Crosswords, variety word puzzles, cryptics http://www.tripleplaypuzzles.com/
Some Image Puzzles
Image puzzle — An altered image — can you figure out what it was? http://sln.fi.edu/qanda/puzzle3/puzzle3.html
On-line Puzzle Hunts (of sorts)
Some of these sites offer prizes!
The Riddler — Online games with prizes http://www.riddler.com/hub.html
Uproar — Online games with prizes http://www.uproar.com/
And others...
WormWorld — math/logic puzzle (Cliff Pickover) http://sprott.physics.wisc.edu/pickover/discov.htm