OpenDBA - A web based Database Management and Monitoring Tool for Microsoft SQL Server. [Open source] - http://www.opendba.info/
EMS Manager - EMS Manager is a graphical tool for database administration and development. Works with PosgreSQL, Interbase, Firebird, and MySQL. [Commercial] - http://www.sqlmanager.net/
Navicat - GUI administration and development tool for Linux, Windows, and Mac OS X. Navicat supports secure remote database connections and Microsoft to Open Source migration. Supports MySQL and PostgresQL. [Shareware] - http://www.navicat.com/