The Theorem of the Day, by Robin Whitty. The content of this page is a hundred single-page things which state a theorem, along with giving a pretty picture related to the theorem and some historical motivation or examples. Whitty writes that "Each day offers a different theorem (or lemma, law, formula or identity), each one worthy of adorning the walls of a mathematical Alte Pinakothek, Guggenheim, Louvre, Tate, Uffizi or Zach Feuer." In idle moments I used to think about making a "mathematical coffee table book" of this sort, but I'm not so good at making pictures, so I blog instead. Now I don't have to.
My favorite out of the ones I've looked at is Ramsey's theorem, because the picture includes aliens; these are the aliens of the famous story of Erdös in which he says that if aliens want us to tell them what the Ramsey number R(5,5) is or else they will destroy the Earth, we should get all the mathematicians in the world working on it; but if they want to know R(6,6) our best chance is to try to kill the aliens.
Incidentally, it's not actually a "theorem of the day", in that there's not a new theorem every day; but don't let that stop you from looking at it. A new theorem is displayed every day, but they cycle back about every three months. How this is implemented (it's nontrivial since new theorems are inserted into the rotation every so often) is explained here.
I found this while Googling for more stuff on Robin's theorem, which I wrote about this morning; Robin's theorem is not one of the theorems listed here. Rather, the Theorem of the Day is maintained by Robin Whitty. The name "Robin's theorem" confuses me when I see it in print, because there are a few things I refer to as "Robin's theorem" since they're due to a mathematician of my acquaintance whose first name is Robin. But I found out that Guy Robin is French, so I just pronounce his name in my head in a very exaggerated French way.