The easiest way to do this is to use the search engine, because it
is not limited to a site  collection (or even web  application or
farm).

Tag the data in some consistent way (e.g. with a standard content
type or with special keywords) so that you can construct a search
which will find the relevant items.

Then, place a Search Core Results web part  on a page and use its
taskpane to configure it with that search so that it always displays
the relevant items. When new data is added, as long as it is
correctly tagged, it will then show up automatically in the web
part, after the next search crawl.

On that note, instead of having to tag the data in a certain way, is
there a way to create a search scope that only returns sites? I
tried creating a scope, and using ContentClass = Site, but that
didn't return any results.

It would be great to use this idea to display a list of all sites in
the farm, if only there were an easy way to filter to show ONLY sites.

This page has a list of all of the ContentClass values:

www.devcow.com/.../the-
contentclass-and-isdocument-properties-along-with-the-welcome-page-
caveat.aspx

Laura, it looks like setting up a scope with a property filter of
ContentClass set to sts_web should to restrict the results to sites.
From that blog post, it looks like the home page of a site  is
treated as the sts_web content  class (other pages are the
STS_ListItem_850 content class, exactly as you would expect.

It seems that you want to collect and view items in a list view. Have
a try "sharepoint list collection". Maybe it can help.
http://www.sharepointboost.com/listcollection.html