WordPress Hooks from Flat Earth is an amazing project to list all of the hooks in WordPress, from version 1.5 to 2.0. The WordPress Hooks project was initiated by Mark Jaquith, with a lot of help from WordPress developers and volunteers.
An example would be when I posted this entry publish_post was fired, and passed along the ID of this post. Plugins could use that ID to do things related to this entry, and pass on the ID for other plugins, but after all the plugins have run, the information passed along (the ID) is thrown away. Actions also have the possibility of not passing on any information (“No Parameters” hooks). These are hooks that are merely placeholders… like a template header or an admin footer. These are the only hooks whose functions should not return any information. Important note: actions are not all “No Parameters” hooks! It is still important to return any information you may get when using an action hook… even though WordPress won’t be using it, other plugins might!
The list is not totally complete. There are still many hooks which are yet to be documented, and experts in WordPress plugins, API, and hooks would be appreciated to help fill in the blanks. There is another related list on the WordPress Codex called WordPress Function Reference.
If you write WordPress plugins, or are thinking about it, this is an invaluable resource.