The SyncBridge software uses Mac OS X 10.4 Tiger's built-in SyncServices to share calendars over the SyncBridge servers, allowing others to subscribe to and edit calendars, sending changes up to the servers and syncing back to your own calendars. Users can even log into the site to see their calendars displayed, though it isn't possible to edit the calendars on the web; that has to be done through an iCal client. A tour is offered at the site, explaining each step of the process.
SyncBridge is charged as a yearly subscription service at £25 / $46, which probably places it outside the range of the casual calendar editing/sharing enthusiast. Nevertheless, this is a viable alternative to waiting for Mac OS X Leopard to land next spring.