The uselessness of pi: outstandingly comprehensive. OK, so I’m a sad case who laughs at maths jokes. That aside, the two best links on that page are both dead, but one can be found: a proof of the rationality of pi, which is great. The other fantastic one is “500,000 digits of the square root of 4”, which is also down. Frankly, I don’t need to see it, but I am still laughing about it. :)