Ok, granted, you'll have to actually pay to use some of these commercially they are still fantastic and my three favorite sources for stock. Check them out:

10 free HIGH RESOLUTIONS a DAY! Photoexpress.com

A nice medley with sizes ranging from 72 - 300 dpi is morguefile.com

This is the free portion of one of the biggest online stock photo communities: Stock Exchange

Hope you enjoy and find something useful!