Try this site

http://www.mapcruzin.com/free-portugal-arcgis-maps-shapefiles.htm

I am pretty sure Portugal has its own SDI portal.  If you are fine with the Portuguese language, then this could be a big help.

http://snig.igeo.pt/portal/

I agree with Eric the National portal to geospatial data of Portugal is

http://snig.igeo.pt/portal/index.php 

http://download.geofabrik.de/europe/portugal.html 

Thanks Himanshu Raj!!! The link really works!! 

Hi Patricia,

DIVA-GIS and GADM are quite good sources of geodata as well. If you are also interested in statistical data (population, etc.) you can try EUROSTAT websites, which might be pretty useful (please see the links).

Regards

Lukas

http://www.diva-gis.org/gdata

http://www.gadm.org/

http://ec.europa.eu/eurostat/web/gisco/geodata

Does anyone know where I can download the map with the boundaries of Census Tracts for Metropolitan Lisbon as a shapefile or GeoJSON?

You can use DIVA GIS