Dave,
"Sharing" can be 3 different things as far as Ical goes.
1) There is sharing calendars that other's can edit. If I share this with you, you get to change it.
2)There is sharing calendars that another can see (that update automatically). If I share this with you, you get to see it, even after it changes.
3) And there is sharing exported calendars (updates would be manual). If I share this with you, you only get to see it as it was when I sent it to you. I would have to send you updates for you to see changes.
So, which do you want to do. In some cases folks want to do all three (or at least 1 and 2.
You have to have .Mac to do #1.
#2 is possible through the internet or locally via a webDAV server (eg iCalShare, iCalX), or through a local network or on a single computer via a 3rd party app such as iCalPublish.
# 3 is possible.
You can do any of these with Panther or Tiger though some folks have reported some problems when trying to sync between Tiger and Panther.
Now about "crazy". I understand your sentiments, but I don't believe you are going to find any "included with the computer" or "free" solutions that offer anything more, unless you want to dive into open source.
Fast-User-Switching. In "System Preferences..." (under the Apple menu), there's an "Account" selection. In there is where you set up multiple users. At the bottom of the Account/User list is "Login Options". There you will find "Enable fast user switching" which allows more than one user account to be logged into your computer at once. Download "Welcome to Mac OS X 10.3" here:
http://docs.info.apple.com/article.html?artnum=50071
and turn to page 9.
So if you need more help we need to know what version of OS X you are on, and what you want to do given the limitations. And you might want to start a new thread as it seems like your path will be significantly different than Peter's. Just the same, you will likely pick up a lot following this thread.