Math/Research
The MathSciNet article database --
Willamette remote login
The On-Line Encyclopedia of Integer Sequences
Discrete conferences, problems, and photos
Math/Organizational
The Mathematical Subject Classification
The Erdös Number Project
The Mathematics Genealogy Project and Grapher
American Colleges and Universities
Math/Diversions
The Mathematical Quotations Server --
My personal favorites
Harvey Mudd Math Fun Facts
MAA Online Math Columns
Macalester Problem of the Week
Math/Humor
Brown Sharpie
xkcd
TeachPi.org
Games/Community
Board Game Geek
Funagain
GameStorm
Games/Play
Blokus
Set
Puzzlebeast