From http://www.workz.com/content/1027.asp

Dimension Guidelines

Currently, size standards for banner ads are really just recommendations. Two professional associations, the Internet Advertising Bureau (IAB) and the Coalition for Advertising-Supported Information and Entertainment (CASIE), have compiled a list of the most popular and most effective banner ad sizes. According to IAB's Web site, a joint committee of IAB and CASIE members "has identified the following as the most commonly accepted":

Type: Dimensions (in pixels): Full banner 468 x 60 Full banner w/ vertical navigation Bar 392 x 72 Half banner 234 x 60 Square button 125 x 125 Button 1 120 x 90 Button 2 120 x 60 Micro button 88 x 31 Vertical banner 120 x 240