Tunity:

Tunity allows you to hear a muted TV’s audio directly to your mobile device. It acts just like a wireless headset – no need to purchase an expensive set.

Scan the TV and let Tunity locate and match the audio and stream it straight to your iPhone or iPad.

I used this app on New Year’s Day during ESPN’s broadcast of the Rose Bowl game (GO DUCKS!) and it worked great. I can see it being very useful in a sports bar with dozens of TVs set to different game broadcasts but no audio to listen to.