HStore [1] is NoSQL plugin for Postgres. Works great. Been available for a long time too.
http://www.postgresql.org/docs/devel/static/functions-json.h...
Also, Posgres already has hstore (already mentioned) and XML columns, which are schemaless and "unstructured".
It's probably not a good idea though ;) You don't get any benefit of indices in that case, and I'm not sure what would happen with nulls.