Developer 2D boy is extending their birthday promotion for World of Goo, the wildly successful puzzle game about the physics of goo. The game turns one-year-old this month. For the "Pay What You Want" promotion, you can enter any price, as low as a penny, when you download it directly from the developer's site.

The promotion began last week. 2D Boy explains how well it's gone over so far:

Since the birthday sale started, about 57 thousand people bought World of Goo off our website. The average price paid for the game was $2.03 a significant percent of which went to PayPal for transaction fees. Normally, they keep about 5% of the revenue, but because PayPal fees are structured in a way that they take a larger percentage for smaller transactions, we ended up paying over 13% in transaction fees. For all purchases of around 30 cents and under, we actually saw no money, PayPal took it all, but they probably ended up losing money on most of those transactions ($0.01) as well, they're not the bad guy.

The promotion continues through October 25th, so don't miss you chance to either stick it to PayPal or financially express your gratitude to 2D Boy. Either way, you get a cool puzzle game out of the deal.