Do a Google search for UserAlerts. This is a free web part created by
Jan Tielens of Lead-It that will allow WSS site admins to subscribe
other users to alerts.

The user  interface is a little awkward, particularly if you have a lot
of users on your site. But he includes the source code, so you can
revise it to suit your needs.

I ran it and it errored out. It told me to check a specific log
but that was blank. I sent Jan an email through his blog. Maybe I'll
hear back? I'll letcha know what I found out.

Also, it might be a little premature to purchase (uless you are only
running WSS) because: (from their forums)

"SPS alert  Manager will be available in the 2nd Quarter of 2006. SPS
Alert Manager will provide SharePoint administrators with the ability
to add Area, Document, Folder, List, Listing, List Item, Person,
Search and Site Registry alerts  to one or more user  profiles. In
addition, the ability to specify an e-mail address that is different
than the recorded address for a user profile will make the SPS Alert
Manager more flexible and powerful than our current WSS Alert Manager
product."

So I'd be interested on the pricing for that puppy when it comes down
the pike.