Mine Bitcoin<number>
Your browser hashes Bitcoin<number>, counting upward from a random starting point and keeping whichever result is lowest. Real SHA256d, real brute force, and the best hash you find can go on the leaderboard.
Random for every visitor, so you are not re-checking numbers someone else already tried.
What this is not
This page runs the same hash function Bitcoin mining runs, against the same kind of odds, which makes it a decent way to feel how the search behaves. It is not Bitcoin mining.
The starting number is drawn at random for every visitor out of a space of 1021, so two people almost never grind the same stretch of numbers. Nothing is lost by starting high: the hashes are just as unpredictable at 1020 as at zero, and the low numbers have been picked over already.
What gets hashed here is the ASCII string Bitcoin<number>, not an 80-byte block header. Nothing is connected to the Bitcoin network, no block is being competed for, and nothing can be earned. A browser manages a few hundred thousand hashes per second where an ASIC does hundreds of trillions, so the gap is around nine orders of magnitude.