SQuirreL, written in java, will work with any RDBMS for which there's a JDBC driver.
It works very well for both sending SQL, rendering result sets, and allowing browsing of database objects (from a tree). For several RDBMS, it includes extensions to automatically generate common statements.
Especially nice is that the query pane executes either the selected sql, or the line the cursor is on up to the first blank line. This allows you to write several statements, and to execute them either individually or in a batch.
And it's free and open source.