I use SQuirreL SQL and I' pretty happy with it. I'm pasting the "Overview" below:
SQuirreL SQL Client is a graphical
Java program that will allow you to
view the structure of a JDBC compliant
database, browse the data in tables,
issue SQL commands etc, see
Introduction. The minimum version of
Java supported is 1.6.x as of SQuirreL
version 3.0. See the Old Versions page
for versions of SQuirreL that will
work with older versions of Java.
SQuirreL's functionality can be
extended through the use of plugins.
A short introduction can be found
here. To see the change history
(including changes not yet released)
click here.
For a more detailed introduction see
the English or German of our paper on
SQuirreL.
Susan Cline graciously took the time
to document the steps she followed to
setup an Apache Derby database from
scratch and use the SQuirreL SQL
Client to explore it.
Quite some time ago Kulvir Singh
Bhogal wrote a great tutorial on
SQuirreL and published it at the IBM
developerWorks site. He has kindly
allowed us to mirror it locally. The
tutorial is not really up to date but
especially for doing the first steps
it is still of help.
SQuirrel was originally released under
the GNU General Public License. Since
version 1.1beta2 it has been released
under the GNU Lesser General Public
License.
It's maybe not the most nice looking application but it does its job, and pretty fast: add or configure your driver, create an alias, and there you go. Actually, it's my favorite Universal SQL Client.