I saw this a few minutes ago and I thought I'd share the news.  Apparently there was a game called Glitch (perhaps some readers are familiar with it; this is the first I've heard about it myself) that ran from 2009-2012.  They've just released a huge amount of art assets into the public domain.  Here's the announcement from their art asset page:

The collaborative, web-based, massively multiplayer game Glitch began its initial private testing in 2009, opened to the public in 2010, and was shut down in 2012. It was played by more than 150,000 people and was widely hailed for its original and highly creative visual style.

The entire library of art assets from the game, has been made freely available, dedicated to the public domain. Code from the game client is included to help developers work with the assets. All of it can be downloaded and used by anyone, for any purpose. (But: use it for good.)

Tiny Speck, Inc., the game’s developer, has relinquished its ownership of copyright over these 10,000+ assets in the hopes that they help others in their creative endeavours and build on Glitch’s legacy of simple fun, creativity and an appreciation for the preposterous. Go and make beautiful things.

Unfortunately, a large portion of their assets are in Adobe FLA format, which makes them difficult for most people to use.  If there are people out there with Adobe Flash (or some tool capable of ripping the content from FLA files), it would be awesome if we could get some people to convert these files to an open format and upload them to OGA.  It's a massively large project, so there's no need to tackle it all at once.  Just do what you can and upload it, and we'll see to it that they're put into a collection so that people can find them easily.

Note: I've upload the PNG sprite assets here.