Set® The best game ever. Deeply mathematical, but you can play happily now knowing that at all. My younger daughter Hannah won the first, and only, SET® National Tournament in 2006, so she’s the only official SET® Grand Master in existence. She’s very good. There is a book in progress, coauthored by the 4 people in my family.
Cap Builder Jordan Awan (Lafayette REU 2013) wrote this to allow you to create maximal collections of points in the affine space AG(n,3) that have no lines. Give it a spin!
OEIS The Online Encyclopedia of Integer Sequences. A000354 is a fun one to look up.
Kali Make your own Wallpaper patterns!
Mathworld Extensive math info.
16 puzzle This puzzle is an excellent way to spend some time.
Free rice Donate rice and improve yourself at the same time!
Charting the Beatles Beatles + math = heaven
Save the words Adopt a word and save it from obscurity.