"My number is bigger!" is a contest held in a discussion thread at the xkcd forums. The thread was begun on the 7th of July, 2007, and as of March 2015, it consists of 1466 posts.
The goal of the contest is to come up with a finite, well-defined number bigger than the last poster's number, by using already established numbers and notations (such as Graham's number and xkcd number) and without referring directly to the previous number. Another rule is that all numbers must be "computable" — that is, their definition must only employ computable functions.