I was recently wondering how to remove the 'Create Site' option from the Site Actions menu because I wanted to have a custom site provisioning routine since I wanted my users to only use a specific sub-site template. I searched all over the place in my farm for custom actions that are created by the installation. I eventually gave up because I couldn't find it.
Luckily, I can check that task off my list now because John Holliday posted a list of the out of box custom action IDs and Groups so we can remove them. He also shows how he found them.
Thanks, John!
PS - He's also got a list of the out of box Fields with their static names and FieldRef ID Guids