Another excellent example of extending PostgreSQL is that of the project PostGIS, which adds a range of geographic data types (aka geometry) but also a plethora of management functions.
These functions including administrative tasks for making geographic data accessible, but also includes manipulation and data creation functions.
Here are some examples of the power available: -you can create a POINT feature in the database
-then buffer the POINT by a certain distance
-then find overlaps of adjacent buffered points
-then find overlaps with LINE and POLYGON features nearby
-then INTERSECT polygon features of interest with other features.
Really starts to look like a full GIS doesn't it? Of course it does have a built in graphic interface - but MapServer can take care of that. The sweetest part is that all your tabular data can be nicely sitting alongside, in the same table, as your geographic data. Now that's great! (Oh yeah, and it's not hidden as a blob that only a vendor tool can access)