Plone Trac contains bug reports, Plone source code and commits. Useful when you encounter a new exception or you are looking for a reference how to use API.
Zope 3 API reference. Good source for up-to-date Zope Python API and ZCML references.
Zope 2 book. This describes old Zope 2 technologies. The book is mostly good explaining some old things, but ‘’‘do not’‘’ use it as a reference for building new things.
Kaizen Plone problem oriented recipes to solve Plone problems.
Plone Cheat Sheet. A single list of every technology you need to learn to develop with plone3, why you need it and how to learn it. A bit out of date, but a good overview nonetheless.