I am sure you have had the Aha! Moments in your life and I had one of those last week when I saw TimeBridge at the Web 2.0 Summit. Scheduling with external customers had been a big pain for long time that both Stephen O'Grady and I have blogged before. This was an opportunity waiting to be snapped and I think TimeBridge is getting there.
TimeBridge has built a Personal Scheduling Manager that works across companies, time zones and calendaring systems. It is a “full service” service, in the sense that it provides help throughout the life cycle of setting up the meeting, lunch or other activity, including collaboration, distributing meeting materials and handling changes. It works for 1:1s as well as larger group meetings.
I played with their beta, it currently only works with Outlook but integrates very well with Outlook client. I could set meeting and anybody (even non-outlook) external users were able to use it through the website. What was missing was the ability to schedule meeting from the web applications itself. I am sure it is in the works.