Compiled 2014-03-10 / Verified 2015-11-11
As noted elsewhere, my total inability to draw has often hindered my creative processes, forcing me to rely on overlong text descriptions or poorly-drawn sketches. While researching other possible graphic sources, I came across several sites brimming with resources for game developers like myself, who wish to use CC and PD art to make new games.
Below are links to some of the sources I've been using, compiled as of March 10, 2014. The links were last verified on November 11, 2015.
OpenGameArt.org - Also listed on the site above, this site includes free sprites, tiles, sounds, and other useful bits.
Game-icons.net - A collection of SVG-based game-related icons that can be used in most any digital (or paper) games.
Ambrosine's Game Creation Resources - A searchable database of game engines and resources, plus lots of info about classic games.