WordPress.org

The main Developer page with options to enable errors and variable output. These settings are set per-user, so no one else will see your debug data cluttering up their screen.

The Options page for easy modification of entries in the options table, inlcluding serialized data. Ability to add and remove options will be added soon.

The Hooks page displays detailed information about addaction, doaction, addfilter, and applyfilters function calls and where they are located in the source files.

Run Query is a simple tool that lets you run arbitrary queries on the database. Very useful for development, testing, and repairs.

The PHP Info page simply shows the output from phpinfo(). It may not be fancy, but it can be extremely useful.

The Defines page shows detailed information about defined named constants. Can't remember what that constant was called that holds the location to the plugins directory? This should help you remember.

The Upload Files page is an invaluable tool for quickly and easily uploading any type of file (including archives) to any location inside WordPress. You can even upload from a remote URL.