There are many ways to view a list  outside of its Site boundaries.

In the browser:
1. Using the Page Viewer Web Part
2. Using the Web Capture Web Part

In FrontPage 2003:
3. Using "Add to Catalog" and the data  View Web Part (DVWP) for Lists in Sites
in the same Virtual Server
4. Using Lists.asmx Web Service and the DVWP for Lists in Sites in other Virtual
Servers

In VS.NET:
5. Using Lists.asmx Web Service and a custom  Web Part for Lists in Sites in any
Virtual Server
6. Using the WSS API and a custom Web Part

My current favorite is #4.

Keeping a SharePoint list in sync with an external  data store  would be possible,
but why? If you want to view external data from within SharePoint, use the DVWP.
If you want access to a SharePoint list from an external application, use the
SharePoint API. There are certainly situations when it is needed, but it is
rarely wise to duplicate data.