Adam Trachtenberg beat me to the punch on announcing our Free_eBay_API_Calls.
Hate to repeat myself, BUT there's honestly nothing even close in terms of the breadth of the web service platform eBay offers to developers (now free!) focusing on product search, transactions and order fulfillment. Our developer network includes over 20,000 developers, 1,600 applications live to site and 20% of eBay.com's 200MM+ quarterly listings generated by 3rd party applications using eBay's APIs for finding, selling, buying, client-desktop, ASP, wireless, interactive TV and affiliate applications (affiliate tools are by far the minority of apps LTS). Since winning the 2005 Codie for Corporate Achievement, Infoworld just announced eBay's Developer Program as the winner for Top IT Project in their Retail category Top_IT_Projects
as well... Alright, fine, I'll stop.
Oh, by the way (as opposed to our brethren), we also welcome commercial applications... (couldnt' stop myself)