Tungle does exactly that. There's no need to switch calendaring apps to use Tungle, either. It can read whatever you already use -- Outlook, Google, iCal... (and according to the Web site, Lotus Notes is coming). Then just mark the hours you are available to meet and send it out.
The recipients don't need to sign up for Tungle; they just choose the best time and send it back. If the invitation crosses time zones, Tungle automatically adjusts. And anyone who does use Tungle automatically find that meeting added to their calendar.
Tungle has an iPhone app, and it integrates with social networking services like LinkedIn and Facebook, where people can check your availability and schedule meetings with you, again, without needing to sign up for Tungle on their own.