You would think that by now we would have a good technical solution to the question "when can you fit me in?". For my coworkers Exchange and Outlook work like a charm. Outlook even has the ability to post my free/busy data to the 'net via ftp, webdav, or locally. However only Outlook understands that file (even though it is part of the iCalendar standard).
I did find a few people at the fringes who deal with this sort of stuff. The most helpful was iFreeBusy.com, but they seem to have a good system for letting my Outlook know if someone who uses iCal is busy, and not for publishing my free/busy data to the web.
One guy who works for a church rolled his own solution, which is downloadable, but requires perl, which I didn't feel like messing with. Not to mention that it doesn't really detail what version of perl, or what modules are required.
Frustrated, I turned to the only possible solution left: hacking. Witness the newest page to davidkearns.com: free/busy. I simply set my outlook to publish my free/busy data to my website, and then load that file, parse it out, build the object, and render it all purty. Add a calendar to select which day, and voila. I spent less time on the solution than all the research that determined I needed to build it myself. Oh well, I now know quite a bit about standard calendar formats...
Update: iFreeBusy.com is already on top of a solution. Try subscribing to my calendar in Sunbird, or another Mozilla type jobby with the calendar add-on. (Though since Outlook can only publish automatically to one place, my homepage wins out and this data will get old).