I created this variation on the Tor onion logo as a t-shirt design. And it got printed on t-shirts. But after that, the Tor community really embraced it and it has now been used as a logo for the Orbot app, and repurposed on a variety of graphic designs (business cards, posters, postcards, etc). Someone even got it tattooed on their arm!

You can get a t-shirt by donating to Tor here.

My initial concept sketch in late 2012: