I know 99.9 percent of people know http://www.smashingmagazine.com -- but for those of you who don't -- they regularly feature free content including icon sets, Photoshop brushes, backgrounds and other things. It pretty much picks and chooses a lot of cool things that stand out on the sites that raghuc mentioned.
Yes, Smashing Magazine is great. I usually find my resources by googling "free vector tree" or something like that, whatever I need. So I usually end up on the sites directly rather than on a site like Smashing Magazine which picks and chooses the best.
This is a fantastic thumbnail image search tool for Flickr. You can change settings to look only for Creative Commons licensed photos (be sure to select for SAFE filtering as well). The Thumbnail will also indicate the largest size the particular photo is. Clicking on a photo opens the relevant Flickr page in a second window.