This list includes sites that offer free and pay-to-use artwork. You will not find places to contract artists for custom artwork. Basically, these are resources that offer the designer immediate gratification.
Unsplash: Free (do whatever you want) high-resolution photos. 10 new photos every 10 days.
Gratisography: Free High Resolution Pictures you can use on your personal and commerical products.
Flickr Advanced Search: Flickr's Advanced search has a similar option allowing you to search through people's photos that have a Creative Commons license, some of which would allow reuse in a commercial product like a game. Just make sure you check the boxes that allow for "Commercial use" and "modification/adaptation".
Dingbat Depot: Fonts that are iconic.
Game-icons.net: An online repository providing heaps of cool game related graphics. They are provided under the terms of the Creative Commons 3.0 BY license. This means you can use them freely as long as you credit the original author in your creation.
Google Advanced Image Search: Google's Advanced search includes a Usage Rights filter that allows you to search for images that allow commercial reuse.
Cliker: Royalty free and public domain clip art.
ClipArtLord: Royalty free and public domain clip art.
MorgueFile: A collection of over 200,000 stock photos that are all licensed under the MorgueFile license, which allows commercial reuse, adaptation and does not require attribution.
Open Font Library: Collection of 169 fonts licensed for commercial use under the Open Font License.
DaFont: A huge collection of beautiful, free fonts. Be sure to check the usage rights of each particular font you intend to use. While many are royalty-free to use, some are not.
Larabie Fonts: Ray Larabie offers free fonts for both personal and commercial use.
Open Clip Art Library: Collection of 20,000 clip art images released into the Public Domain.
Open Game Art: A collection of video game and board game art made available for free or attribution.
Spiral Graphics: offers free versions of their high-end texture-creation software. They provide many modifiable and customizable textures that you can use in commercial projects royalty-free (see usage rights here). Both Genetica Viewer and Wood Workshop are amazing tools for creating tileable, high-res textures, the latter for wooden textures, the former for almost everything else.
Stock.Xchng: Another collection of over 350,000 stock photos that are licensed for commercial use as long as you are not selling the images themselves (e.g. - a T-shirt with the image on it). Using the images to enhance other creative works such as books, websites, or games would be OK.
Wikimedia Commons: Over 5 million public domain and Creative Commons licensed images.
L3DT (Large 3D Terrain Generator): Free version has some limitations on map size, but otherwise a very good program for generating textured 3D terrain, height maps, and other things. Great for creating realistic landscapes.
30 Free Photo Sites: This is a quick list of 30 sites that have free photos.
Free Art (created by The Game Crafter Community)
Note: Be sure to check the individual licenses for each artist so you know how their art can be used.
Free Poker-Sized Card Templates from Mike Wokasch - TGC community member, Mike Wokasch, created some free-to-use, full bleed poker card designs that you can use with The Game Crafter's print-on-demand service.
Pay To Use Art
Pixmac: The Pixmac Picture Market offers stock photos and vector illustrations for all of your printing needs. Prices are very reasonable and they also have over 45,000 free images available.
RPGNow Stock Art: RPGNow is a PDF publisher that offers a decent number (877) of reasonably priced collections of stock art that can be reused royalty-free in your game once you purchase a license. Great resource if you're looking for some cheap fantasy art (which is often hard to find at "typical" free image repositories).
Photos.com: A couple million stock photos as cheap as a couple dollars each.
Clipart.com: About 10 million stock clip art images.
Getty Images: The largest purveyor of images in the world. Also sells music and and stock video footage.
Fotosearch.com: 32 million images starting at $3. Royalty-free clipart pictures, illustrations, video clips, posters, prints, and more.
Shutterstock: Possibly the best stock photo provider out there with over 16 million images in its library. Also sells stock video footage.