From flamedfury.com: An 88x31 button is a small graphic image that is used to link to another website. The buttons are called 88x31 buttons because they are 88 pixels wide and 31 pixels tall. These buttons were commonly used in the early days of the web to promote websites and to help people navigate from one site to another.
This list is by no means exhaustive... if you're on this site you probably have some means of contacting me, so if you have a button just send it my way and I'll put it up here. :-)