3) OpenPhoto
http://www.openphoto.net/

The Open Photo Project is a stock photo community and framework devised, implemented and supervised by Michael Jastremski. Its purpose is to allow photographers to share and protect their works through Creative Commons licensing where you can view photo thumbnails by license.

The site uses a wiki to manage its collection. To contribute to the site or to see other features requires registering. Click Spy to see what photos other users are admiring or find pictures by using the categories or the search tool.

Key features:
Self-referencing stock photo database
Integrated licensing engine
Open source-like platform (interface is similar to Wikipedia)
Your own (username.openphoto.net) website
Opportunity to gain exposure for your work
RSS feeds for knowing when new photos are added

2) PixelPerfect Digital
http://www.pixelperfectdigital.com/

PixelPerfect Digital is an image archive with over 4,000 images sorted in 146 categories free for commercial and non-commercial use. The newest files appear on the gallery home page and randomly selected images on the bottom of the page. You can stay on top of the newest files by subscribing to the RSS feed.

Its license states, "You are permitted to use our products to design as many projects as necessary, including those you are creating for your clients. However, beyond this use, you may not resell or redistribute our products in any other way, modified, unmodified, standalone or combined with another product without prior written (email) consent from us."

Browse photos by category or use the detailed search page, which offers many options. Also you can browse by Contributors or check out the Top 25 Images. The details for each picture include the photographer's name, size, number of views and downloads and the average rating. Click on the picture to view a larger size and more details including its description, keywords and EXIF details. A nice and well done site.

1) morgueFile http://www.morguefile.com/

morgueFile contains free high-resolution digital stock photographs and reference images for either corporate or public use. The official purpose of the morgueFile image resource is one of providing free image reference materials for all creative pursuits.

The archive contains over 55,000 digital images, which can be searched and accessed through well-organized categories.

morgueFile provides access to a broad set of image categories, organized under these main sections:

Animals
Objects
People
Scenes

Or you can search it through a simple and fast responding interface.

While some people will find the morgueFile thumbnails too small (and I agree in good part, but at least it gives us a choice), there are two good things that make it worth overlooking the small thumbnails:

a) search results display quickly

b) though hard to believe these are from non-professional visual communicators, the ones you can view at such small sizes are outstandingly good photos at standard dimensions. So, the small size filters out subjects that do not have strong contrast, a clear subject and poor color saturation.
I personally find the morgueFile contents, its interface and overall quality, most suitable for my needs, and would easily recommend it to any colleague for the general good quality of the available visuals.