For MySQL users, phpMyAdmin -the popular MySQL admin tool-, is probably one of the most-used apps unless you are fan of a desktop MySQL admin tool.
MyWebSQL is an exciting open source alternative to both as it is a web-based MySQL (and SQLite) manager with a desktop-like functionality.
It is built with PHP and, with the help of a completely Ajaxed interface, tasks can be accomplished pretty quickly.
Creating, editing and deleting tables or records are easily done in a grid mode which also makes working with multiple items at the same time possible. Also, shortcuts for any actions exist inside context menus to make browsing faster.
There are other must-have features like import-export, SQL editor (with syntax highlighting), repair tables, search, process manager, etc.
MyWebSQL is a multilanguage app with lots of ready-to-use languages and has a themable interface.
Imho, what is missing the most is the ability to manage multiple server connections with a single login and scheduled tasks (which I hope would exist in the future versions).