There is always pgAdmin (http://www.pgadmin.org/) which is very close to the Postgres core development and thus is always updated to support the latest features of Postgres.
It suffers a lot from the has-to-work-on-all-platforms UI design issues though (even though it uses wxWidgets).
Personally, I hardly ever use the GUI tool (the exception is explaining really big query plans) because the command line psql client is really, really good. It has an awesome line editor and so many convenience features like always shelling out into $PAGER for big result sets, or the very handy \x mode.
And it offers tab-completion not just for internal commands, but even for the SQL queries you are typing.
