math
- Desmos is an amazing online graphing tool. It also has a nice geometry tool.
- Wolfram Alpha, the "computational knowledge engine", can handle all kinds of mathematical input.
- GIMPS, the Great Internet Mersenne Prime Search, is an organization devoted to finding large prime numbers. You can help them out by downloading some software to run on your computer.
- Number Gossip gives you special properties of numbers from 1 - 10000.
improv
- The official site of the Canadian Improv Games