Anki flashcards. Anki is a free flashcard program which automatically adjusts the length of time before you see a card again.
Carleton College Mathematics Department. My alma mater!
University of Pennsylvania Mathematics Department. Where I am now!
Hampshire College Summer Studies in Mathematics. A wonderful summer mathematics program for high school students where I first discovered what mathematics was all about.
The On-Line Encyclopedia of Integer Sequences. Whatever combinatorial object you are counting, OEIS probably has the sequence.
Math Overflow. This is where serious mathematicians go to answer their questions.
Wikipedia. This is where I go to answer my questions.
Wolfram Alpha. From the makers of Mathematica, this "computational knowledge engine" can graph functions, tell you how many homo sapiens have lived on earth, and everything in between.
Shirt Woot. Every day they sell a different shirt; I have bought 20 shirts to date. It is one of my life goals to design a shirt on Woot.