Pet project: Number Gossip
Fun. I learned a few interesting facts ;-)
For instance, I did not know that an evil number has an even number of 1's in its binary expansion.
http://www.numbergossip.com/list#evil_numbers
However, it seems that 666 is not an evil number.
Neat, but you really should provide some exploration entry points.
http://www.wolframalpha.com/ does a pretty good job of this w/ the rotating suggestions and the "Examples >>" link.
This is delicious!
Rare Properties of 93
Cake
The n-th cake number is the maximum number of pieces a (cylindrical) cake can be cut into with n (straight-plane) cuts.
Unfortunately, not everybody gets the frosting. If you cut pizza rather than cake, you get lazy caterer's numbers.
..., 26, 42,64,93,130,176,232,...*3435 deserved more than just: http://www.numbergossip.com/3534
From this post: http://news.ycombinator.com/item?id=2477798