Accessing a database form another database

I've got some GIS tables that I want to use across several projects and
currently I've got each project in a separate database. So say I've got
two databases, one for the Spacely Sprookets project and one for the
Cogswell Cogs contract, but I want to access my WorldMap tables in my
GIS database to make maps for both of these projects.  How do I
reference a table that's in another database? Or should I organize my
PostgreSQL data differently?

Thanks,
- Bill Thoen