<p>I have an announcements list on one site. I want to add it as a web part to the top of each subsite. How can I do this in MOSS?</p>

I've used the Data View Web Part in this case.  Create a web service data source to get the data from the other site's list.
Much like this:
http://www.sharepointblogs.com/ssa/archive/2007/02/23/showing-web-service-data-in-a-data-view-web-part.aspx