There are good list of free stock photography website on the web, but i wanted to make my subjective list of my own best top 5 free stock photography website for designers. For my work, i had to browse so many of this sites until i decided, which ones are really worth it, so now i share my view on the top best free stock photography websites that will be of use for you.
While most of this sites want to sometimes lure you into one of the paid stock photography website, you can find some gem out there. Of note on some of those websites you need to register to download, so be sure to read their privacy policy. Also always read the licensing for each free photos you download, they might be free but you have to obey to the terms of usage.
1. stock.xchng

This is one of the most famous since 2001, but still the best one to find images and also vectors illustrations. They now have over 390,000 images with more than 30k photographers contributing.
Be sure to read the license terms as they differ from one image to another.
You need to register to download.
Images can be downloaded at 300dpi in high resolution.
You can search by : keywords, categories, most popular, most recent, search box.
They also have an advanced search to filter with min/max resolution, date, photographers and type (photos/graphics.)
You can create a favorite list/lightbox.
2. Free Stock Photos.Biz

They have only a bit more than 14,000 free images, but some of those photos are really good, on par with any paid services. The business section is really interesting as it's often what lacks in good free stock photography and what we as designers need the most for our corporate clients.
Read the image license first (they have public domain, creative commons, and custom agreements.)
No registration needed.
You can download image at small size (ie. 400 x 336) or large (ie. 3312 x 2784).
You can search by : keywords, categories, most popular, most recent, search box.
They also let you view all the photos from a same set (great to get some consistency when using stock images), similar images (by keywords), and more photos with the same person (if you are looking at a photo from a person).
3. Stockvault

They have more than 26,000 images in 11 categories and more sub-categories. They have a good wide range of picture and an easy-going visual websites. I like particularly the industrial category as it is quite hard to find free usable images for this type of businesses.
All licenses are Creative Commons attribution license.
No registration needed, but you can register to add to favorites and get a less advertising driven website.
Images download go from medium to high resolution (3888x2592).
You can search by : keywords, categories, most viewed, most downloaded, search box.
Sort categories by: size, date, viewed, loved…
4. Freerange Stock

This website is hit and miss, i found some of the best free stock photos for my collection but you have to dig a bit more than on the previous website mentioned to find them. They have user uploaded photos and images from the in-house photographers (which are usually of good quality).
License is pretty open so that you don't have to worry (still read the details in the Support>Faq section).
You need to register to download.
Images are generally high resolution of 2400 x 1600 (in-house photographers), user uploaded photos range from medium (1600 x 1200) to high (3888x2592).
You can search by : keywords, newest, popular, artist, search box. No categories however.
5. Morguefile

Morguefile is a classic around here, since 1996! A lot of free quality pictures. However it tends to take more time to find the really great shot you need than some of the others.
But take in mind that if you didn't find that particular image from the sites listed previously, morgue file will usually have a very broad selection that might save you this time.
No registration needed.
Images are high resolution.
You can search by: search box.
Filter by: categories, size, date, creative, rating, colors, geotag.
Sort by: Total downloads, total views...
Alternative
Here are three alternative websites that are good also:
Free stock photo search engine
To find your free photos and get a good overview of what you can find, it is useful sometimes to use search engine that aggregate free stock photos from several website.
Here is a list of three websites:
It uses google search with the OR operator to make a custom search into 7 different websites. You could do the same with your own custom query.
Here is mine for my top 5:
yourkeyword +(site:www.sxc.hu OR site:www.freestockphotos.biz OR site:www.stockvault.net OR site:www.freerangestock.com OR site:www.morguefile.com)
They aggregated indexes of photos from stock.xchn, Freerange Stock, Morguefile, Flickr, NASA… They have different filter that makes this a quite useful tool.
3. Photoree
This website indexes mostly Flickr images, what is neat about it is that you can filter by license.
Creative communs photos and other
Another great way to find great photography is to look into creative communs photos repository.
Here is a list of three great ones:
Pick the license category you want, do a search, and sort by Interesting to get the best results.
If you need to find an image of a very specific animal, event or something else, it is sometimes the only place. Don't be too ambitious on the quality however.
Here you can find some really gorgeous pictures that are handpicked by the deviantart users. Be sure to read any specific license agreement when you find what you want.
Free stock photo from paid sites
Not only free stock photography website are generous, the paid ones too.
They offer pictures on daily, week.. basis.
Here is the list with the direct link to the free section:
1. Istockphoto
2. Fotolia
3. Dreamstime
4. Shutterstock