5) PDPhoto
http://pdphoto.org/

PDPhoto.org is a repository for free public domain photos. It contains about 2,000 photographs under a many categories. One person has taken most of the photos.

Unless something is clearly marked as copyrighted, you can assume it is free to use. But if you intend to use an image from here for commercial use, please be aware others may have rights that could be actionable. Specifically, you should assume there was no model release. And pictures featuring products or property should be used with care.

You can easily find out whether or not it is public domain by clicking on the photo and viewing its Creative Commons license. The home page links to newest photos, Jon's (person behind the site) favorite photos, most viewed photos and most recent comments. The site has a forum that resembles a guest book or a list of comments.

4) Image*After
http://www.imageafter.com/

Image*After is a free online photo collection. Download and use any image or texture from the site to use in person or commercial work per the terms of use. At the top is a drop-down list for browsing images and textures by category or you can use the search box. This well-organized library can be sorted in different ways and the thumbnail pages include the image's dimensions, number of hits and file size. Plus, you can select how many images to display on each page and even make the thumbnails larger by clicking on the magnifying glass.

Clipping is a nice feature that you rarely see on free photo sites. Click on the scissors icon to "clip" the photo and it saves it so when you're done clipping, you can review all of the clipped images in one page by clicking on the bigger scissors icon next to the die icon. To "roll the dice" and get random photos, click on the die icon. After selecting an image to view it, a box in the upper left corner of the image is for opening the image to its full size.

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.