I recently read a post on Logic & Emotion that discussed how Comcast has began offering customer service.
This is a genius way for companies to leverage Social Media to improve their business. Does it take much to have a couple of your employees in the call center monitor a twitter account while they take calls? I doubt it.
Courier companies - allow users to send the tracking number for instant updates.
Airlines - allow customers to tweet flight numbers for updates (delayed to 12:15, on-time, etc).
Car manufacturers - Road side service?
Movie theatres - movie times. Sold-out shows.
Realtors - post your twitter name to ads, for sale signs to instantly respond to requests.
Nightclubs - provide inquiries into lines, take table service resos.
AA - support over twitter
Children's Help Phone - see above.
Find me on twitter (pcrowe) if you want any thoughts about how you could use twitter with your business.