If you like your database loosely defined, take a look at PostgreSQL’s hstore module, starting around version 8.3:
http://www.postgresql.org/docs/8.3/static/hstore.html
After enabling this module, you can add an “hstore” type column to your table, then store any number of key/value pairs in that column, as long as each key and each value is smaller than 64k.
This in itself is not that big of a deal, you have always been able to store stuff in columns, but hstore allows searching on keys!