This is pretty unique:
http://www.asovision.com/mousou-controller/xevi.html
Basically it's a small keychain controller (w/ d-pad and 2 buttons) that lets you play Xevious with just the sound. If you're good enough you can make it to the bosses and clear the game. There is also a rapid fire minigame.
"Mousou" translates to "fantasy". Judging by the pic of the man playing the thing on their website, you're supposed to close your eyes and dream of the graphics. Their motto is "you can enjoy these games just with sound!".
They also have other ones, including Street fighter II:
http://www.asovision.com/mousou-controller/index.html
http://p-bandai.jp/moucon/
888 yen a piece. The Xevious one lands on March 20th.
That seems neat as a novelty but I can't imagine it playing well.
Good memories just hearing the sounds though. Probably dating myself, but Xevious was one of the first shmups I really enjoyed in the arcade.. and PCE Xevious was the first Japanese game I ever imported.
Wow, that is a pretty out there idea! Can't believe they actually thought of this, haha.