Wikimedia Commons. For a number of open-licensed UIs I've used the Wikimedia images to help create a look that implies I spent an awful lot of time and energy. These icons vary in quality because they were uploaded from different sources, but you can get some of the best icons available from this site (along with, of course, the best freely licensed photographs and other media). Commons is a gem, and if you don't use it, you're probably missing out.
Crystal Icons from KDE were uploaded as PNG, but the original SVG icons can be acquired from their original site. These are very clean and plastic-feeling icons for everything from the power button you see on the left to various arrows to hands to gears. Because they were uploaded in fairly high resolution, using the PNG versions isn't all bad, but you'll still get a better final result if you work from the SVG.
Other icons include arrows, globes, hands and many other cateogries. You can browse all of the categories from Wikimedia's "icons by subject" page.
To use these icons, I recommend always selecting SVG format images and then using Inkscape . Load an SVG icon up in Inkscape and select File, Export Bitmap... to save it as a PNG file, setting the height and width to exactly what you want. For now, PNG is the way Web browsers deal with icons the best, though in the future, SVG will be taking over that role as older browsers fade away.