What are the most popular ad sizes for online advertising?

Nielsen regularly posts a list of the most popular advertising unit sizes. Here is the most recent list: Net Ratings reports the most popular ad units during February 14-18 2008 period.

Dimensions - Unique Ads

Leaderboard (728x90) - 21,081

Medium Rectangle (300x250) - 16,577

Wide Skyscraper (160x600) - 11,442

Non-Standard Dimension - 8,145

Skyscraper (120x600) - 3,082

Full Banner (468x60) - 2,845

Large Rectangle (336x280) - 1,748

Rectangle (180x150) - 1,546

Button #2 (120x60) - 1,329

Square Button (125x125) - 1,240

Other - 4,071