Last week when we were getting ready to go to New York, I wanted to find out what tickets were available at the TKTS half-price ticket booth in Times Square. I went onto twitter.com and entered TKTS in the search box. Within moments I found a Twitter stream (it turned out it was from a TKTS employee) listing the shows available that night.
Very convenient and very helpful - it told us the show we wanted the next day (A Little Night Music) was available. Turns out it was only 40% off, not half-price, but still.
Real-time information on a subject that I needed. Helpful.