More than just mathy, it's also sciency and geeky. XKCD is a must read! When you visit the site, be sure to mouse over the picture and hold. The alt text will soon appear with a hidden punch line and commentary. Here are two of my favorites on:
Correlation and causation
Andromeda Yelton (@ThatAndromeda) keeps a fairly complete directory of XKCD comics by topic called DCKX - Directory of Curricular Knowledge in xkcd. Handy for those who teach math and science.
Mathy Comics - SMBC (Saturday Morning Breakfast Cereal)
Mathy Comics - XKCD