Many of the microstock stock photography sites have some photos that are available for free. The sites offer these images as incentives to attract visitors but there is no requirement to do anything more than sometimes register on the site in order to download free images. The advantage the microstock sites offer over many regular free photo sites is the availability of higher resolution images and a generally higher quality of photos.
Sites that have a searchable collection of free images
These sites maintain seperate collections of photos and illustrations that can be searched and/or browsed. They therefore offer a very useful resource in your hunt for a free image when you're after something specific.
Dreamstime - Dreamstime have a very user friendly searchable free section that offers high resolution downloads.
123rf - 123 Currently offering over 25,000 free images! Images stay in the free collection for 30 days so the content is constantly being refreshed.
FeaturePics - Themed collections of free photos created by FP contributors.
Bigstockphoto don't actually promote any free images on their site but they do make them available to site developers. Scroll down to the bottom of any page on this blog and you'll find links to search and browse the free photos you can get from Bigstockphoto.
Sites that offer regular free images
These sites only have from one to a small selection of images available for free at any given time. They aren't, therefore, great for finding a free photo for a particular use when you need it. Visiting these sites regularly though will enable you to build up your own collection of free photos for potential future uses.
iStockphoto - iStock have a free image of the week and these are archived so at any time there are several available for download. Arguably some of the highest quality free images available.
Shutterstock - Offer a free photo and free vector each week.
Crestock -A fresh free photo is served up every day, and there is an RSS feed if you'd like to subscribe.
Fotolia - Have a large database of free images from which a different selection are available each day. Update; Fotolia now encourage you to connect with them on their facebook page to access their free images.
So there is a wealth of good quality imagery available for free from the microstocks, and if you can't find quite what you need their prices for regular images are of course micro cheap!