The best places to find free photographs for commercial use
Online marketing is all about using great content and, usually, this means great images. Where possible, I always recommend using photos unique to your brand, however there's not always the time or opportunity to get this content and so you may find you need to use stock photography.
You can use one of many paid stock photography sites such as Shutterstock or istockphotos, but if you don't have the budget, there are quite a few websites offering free photos which can be used for commercial purposes (always the check the photo's individual copyright/license details before using as sometimes they do require attribution and things may change since I wrote this!).
Easy to search, high quality images on a wide range of subjects.
Regularly updated, good collection, find more 'arty' type shots here and it feels more creative than many sites.
Large collection, easy to search, will also find some graphics/illustrations.
Good selection, high quality stock photography.
Not a huge collection, but if you're after food photos it's a great place to start.
Free photos and this site also has video for commercial use.
Not easy to search, but this site has vintage photos that are free to use.
A common problem I've had with the above sites is a lack of diversity, if you are looking for an ethnically diverse selection of photos try Nappy (its collection is still growing) and more specifically Women of Color in Tech for free photos of... well Women of colour in tech!
Finally...
If you still can't find what you're looking for from any of the above resources try an image search on Bing and there's an option to select 'All Creative Commons' from the drop down License box, most of the images returned will theoretically be free to use (but double check with the owner before using).