I use the same sites over and over in my prototypes, and while I credit individual contributors there’s rarely a chance to mention the sites as a whole. Let’s rectify that here.
Freesound.org is a great resource for sound effects. It’ll rarely have exactly what you want, but it probably has something close enough to be worth editing into the right form.
If you’re looking for music rather than effects, the Free Music Archive is just the ticket. Aesthetic preferences make it harder, in my experience, to find “just right” music than SFX–and of course, editing music is more difficult as well. Nevertheless, it’s a valuable resource.
Art isn’t trivial to come by, but I get a lot of use out of Game-icons.net. Easy-to-parse and attractive is what I want for prototypes.
When it’s time to use colors (e.g., for layouts), Adobe has a database of attractive color swatches. Their tool for automatically applying color theory is remarkable, too.
Font Squirrel has become my go-to site for typographical assistance. It gets bonus points for making it easy to tell up front how the different fonts are licensed.
All of these sites have been very helpful to me. I highly recommend giving them a look; I expect you’ll find them the same.