After reading Presentation Zen I became interested in finding places to search for free quality images online. Last week I realized that I had a pretty nice list saved in my bookmarks and it seemed a shame to keep that to myself. This list does focus on free, public domain or Creative Commons licensed images, but I have nothing against paying for quality stock photography either. As a matter of fact I've purchased some credits over on iStockphoto that have come in handy on occasions when I can't find exactly the right image using other sources (which is also a highly recommended site if you don't mind paying a little).
But focusing on free stock images I often don't need to go as far as stock.xchange. I've had my account on stock.xchange for a long time, but I recently "rediscovered" it after I read Presentation Zen and was interested in good stock photo sources. There are plenty of free high resolution images to download, and some of them of excellent quality. Searching the site is pretty easy, and you are able to favorites to your account.
If you're looking for public domain images try morgueFile, the images here are free and completely available for use as stock photography. There's keyword search as well as an advanced search.
Another good free search is Image * After. I recommend Image * After for stock photography, but it's also a good source for free textures.
The Multicolor Search Lab is an excellent search tool for discovering images based on color, and with a little advanced trick it's easy to get the exact color you're looking for too. Let's say you want images for specific color in your palette, just embed the hex number at the end of the search string: http://labs.ideeinc.com/multicolour#colors=201974; You can even add more than one color for even more control over the color options: http://labs.ideeinc.com/multicolour#colors=201974,842959; You may add up to 10 colors in the search. The images are from alamy stock photography which does include some licensed as well as royalty free images.
Then if you're looking for a meta search site, everystockphoto includes some of the ones I already mention including Flickr and a few others I didn't mention.
Of course Flickr is an excellent source of images, but if you decide to use images from Flickr it's good to understand and respect Creative Commons licensing when choosing photos. What's nice about Flickr is that its API allows many developers to create some pretty awesome search tools based on the site.
One very cool search tool is the Content Based Visual Image Search. This tool unfortunately doesn't allow you to narrow your search by license type, so it's very possible your searches will yield a photo with a restrictive license, but it's a great site for showcasing what's possible with the Flickr API. The search tool lets you browse by tag, and when you find an image you like you can search again by that image's theme or by color/texture. I found it fun to play around with the options, it's too bad there isn't more options for refining the searches though.
Flickr.Storm is a Flickr search that does let you filter by license. I find the interface a little strange, but it does a good job and returns a set of nice images based on tag from Flickr. The advanced search allows you to narrow to a specific Creative Commons license which is useful if you know exactly what license will suit your needs.
As Flickr searches go I definitely saved the best for last though, and that is compfight. The quality of images returned are excellent, the interface is excellent, and it will allow you to filter for Creative Commons. There's an added feature of a blue band that pops up when you mouse over the results that displays the image's original dimensions, very helpful if you're looking for an image of a specific or relative size.
I really found this list helpful when searching for images for presentations, and I've started using it more in basic web designs I'm considering for my class and personal use. Even if you're looking for something to spice up a blog post, you're not going to have too much trouble finding the perfect image from one of these sites.
-UPDATE-
I closed the comments to this post recently due to comment spam, but @mir_b gave me a link to Roy Tennant's FreeLargePhotos.com site. Thanks mb and thanks to Roy for putting this together! Definitely another great site for the list.