The library has access to data from the ESRI maps and data collection, which includes many types of base data. This data is available on the GIS workstations at the Earth Sciences & Map Library.
Open Street Map is also a great source for street data, and other infrastructure layers. It provides great detailed global coverage, especially for outside the United States. It is edited and updated by users, though, so it may be worth validating against current imagery in a GIS program. You can download OSM data directly through the map interface, or from a host of other sources (including Geofabrik and Metro Extracts).
GIS data is available at a range of administrative levels -- from local to state to national.
The following is just a partial list of local government websites containing GIS data.
The UC Berkeley Library subscribes to a database of current parcel data for counties throughout the United States. You can access the database through its Oskicat record, below.
GeoData@UCBerkeley is the UCB Library's geoportal to GIS data. It is not comprehensive, but is a good place to start your search.
Search Oskicat (the library catalog) for GIS data.
Pro tip! Search for [area] gis or [area] geospatial. Start specific, but you may need to broaden your search to the state or country to find everything. Here's an example of GIS data the library has purchased:
Many countries make GIS data available on the web (exampled from France and Mexico are listed below). Sometimes data will be available through the websites of smaller administrative units, independent government units, or through NGOs.
Geolode.org is a crowd-sourced partial list of data portals.