To accomplish this you will need to use the Internet Free/Busy feature in Outlook. You will also need to have a hosted site that you can publish the Free/Busy info to. External users will have to use Outlook to view the free/busy information. I'm not sure of any other method of publishing Free/Busy information.
More details on publishing and viewing the information can be found here:
http://support.microsoft.com/kb/291621
Hope this helps.
-SW