Here are some search engines for free stock images:
Google Images (be sure to set the license options correctly under the advanced search)
Vast image resource for textures. The first place I'm visiting when looking for textures.
Conclusion: Highly recommended.
SXC is a friendly community of photography addicts who generously offer their works to those who need them free of charge. It's one of the biggest free image sites out there with 350,000 photos and counting.
The giant. You can use the advance search to search for creative commons license pictures that can be used free of charge and altered.
Hosts tons of free images for inspiration, reference and use in creative work, of course also for commercial use. It also has a bunch of interesting search options, like the color picker. It has a quite powerful search with lots of filter options.
It has a huge amount of photos with some high quality ones as well. All images are licensed under a Creative Commons attribution license. There is image tagging, but searching categories might lead to better results. Also there are a lot of ads on the site.
Conclusion: Decent site.
They strive to provide high quality free stock photography for commercial and non-commercial use. And there are already some very good photos available.
Conclusion: Decent site.
Public Domain images from the NASA. If you need space, nebula and planet images you might find it very useful. More "image of the day" links can be found here: http://apod.nasa.gov/apod/lib/aptree.html
They also have a website for more details on their copyright and image usage.
Conclusion: Recommended for space images.
The stock image category of deviant art. You can find high quality images here, but be careful about the license restrictions since they might vary from artist to artist.
Conclusion: Recommended, but you have to be careful about the copyright licenses.
Compared to other sites, there aren't that many photos on it. Still, pixelperfectdigital has a good amount of high quality photos. Sadly it also has irritating advertisement and resource-links to other sites that aren't free, like dreamstime.
Conclusion: Go here if other sites do not have what you seek.
Since the search seems to only take the file name under consideration, you're better off using the browsing function. Which makes finding particular images tedious. Also there are a lot of advertisements on the site. The categories are well-arranged with a fair amount of high quality pictures under a creative commons license.
Conclusion: Not Recommended if you are searching for a particular image, but the image quality and pool is good.