Exchange currently has the Scheduling Assistant in Outlook and OWA, but unless you have Exchange Federated partners in order to access each others organizations Free/Busy information, it's limited to internal users. This makes planning with external users more cumbersome.
Basically something like Doodle.com, within Exchange but without the need for federation or extensive preparations. Inspired by a suggestion by Mike Books.
There are several ways to approach this and I've probably not thought this out completely, so let the discussion begin ;-)