A couple of the plugins listed here show up on Extend with warnings about not having been updated in the last two years. You can ignore those warnings. They haven’t been updated because they were written correctly in the first place.
Best Plugin for…?
Great big list of best plugins in several categories (forms, user management, security, etc.)
Post Formats UI plugin makes it easier to add posts in different formats
Custom Fields and Meta Boxes
save yourself a lot of trouble and use the meta boxes class to create your custom fields
Custom Post Types and Taxonomies
Developing Themes and Plugins
- List all hooked functions
- Hook Sniffer
- Show Template
- Debug Toolbar
- Debug Bar Extender
- Debug Queries
- Debug Objects
- Log Deprecated Notices
- Theme Demo Bar
Building a theme (old, but still good)
Theme development training wheels (newer, but somewhat less comprehensive)
Script libraries included with WordPress (spoiler: including jQuery, in compatibility mode)
The basics (note the first comment)
issue tracker ($)
job board ($)
Changing WP’s Default Stuff
Influential Sites Using WP
Importing to WP
Documentation, Discussion and Help
News and Events
Things to Avoid
WPMUDev plugins aren’t inherently evil, but the “one-time” purchase is actually a subscription that will bill you monthly unless you cancel — and by canceling you lose access to updates and support. Use plugins from the official repository instead.
Thesis is a theme framework that replaces many built-in WP functions and hooks with its own. This is inherently evil, in that it breaks plugin compatibility. Use Genesis, Headway, Thematic, Hybrid, or just about any other framework instead.