From PostgreSQL wiki
Many database systems provide sample databases with the product. A good intro to popular ones that includes discussion of samples available for other databases is Sample Databases for PostgreSQL and More
One trivial sample that PostgreSQL ships with is the Pgbench. This has the advantage of being built-in and supporting a scalable data generator--you can make databases of any size ranging from 16MB to 600GB (approximately) with the current version.
The latest collection of PostgreSQL compatible database samples is at PgFoundry Sample Databases. It includes three commonly used benchmark databases:
- World: Based on the MySQL World sample. Has a list of Cities, Countries, and what language they speak.
- dellstore2: PostgreSQL port of a database-neutral e-commerce test application developed by Dell. The original code supports three size scales in their data generator (10MB, 1GB, 100GB), currently only the normal, smallest sized data set has been ported to PostgreSQL. PostgreSQL 8.4: Windowing Functions uses this test data to show some advanced queries.
- Pagilia: Based on MySQL's replacement for World, Sakila, which is itself inspired by the Dell DVD Store.
There are some other sample databases there as well, such as a USDA Food database and a large list of country data via ISO-3166 standards.
- Book Town - used for the examples in Practical PostgreSQL
- Benchmarking databases such as DBT-2 or TPC-H can be used as samples.
- Freebase - Various wiki style data on places/people/things - ~600MB compressed
- IMDB - the IMDB database - see also http://code.google.com/p/imbi/
-  - US federal government data collection see also sunlightlabs
- DBpedia - wikipedia data export project
- eoddata - historic stock market data (requires registration - licence?)
- RITA - Airline On-Time Performance Data
- Openstreetmap - Openstreetmap source data