Awesome

Awesome Odoo

A curated list of awesome Odoo resources. Inspired by awesome-python, itself inspired by awesome-php.

Contributions are welcome - check the guidelines.

Technical Memento (pdf) is a short reference, a little outdated, but that still can't be missed.
Current official documentation actively maintained by the R&D team.
New API Guidelines, by Nicolas Bessi, can provide additional insights not yet covered in the official docs.
OpenERP Tutorial: Module creation and modification of the Point Of Sale, by Thierry Goodin, great tutorial.
Data Model Documentation with UML diagrams.
Odoo HOW TO A list of common forum questions.
The Hitchhikers guide to Odoo A guide for getting started.
Blogs A curated list of several blogs.

Documentation for older version can sometimes fill a few gaps in the latest docs:

ERPPeek provides both a fully featured low-level API, and an encapsulation of the methods on Active Record objects, plus helpers to administrate the server remotely (docs).
Mandibule, an Odoo Model browser (doc).
OdooRPC is a Python JSON-RPC library which aims to provide an easy way to remotely pilot and manage an Odoo server.
Odoo Code Search. A searchable index of odoo source code.

Pycharm a great IDE with Odoo fast development templates.
Sublime Text provides many useful plugins for ease development in Python and Javascript.

Aeroo Reports: a versatile reporting engine with Open/LibreOffice integration (doc).
Pentaho Reports: run Pentaho reports in Odoo and design them using the Pentaho report designer (doc, presentation).
Jasper Report Server: Compose report with iReport or JasperStudio and defined printing action in odoo to retrieve the report seamlessly.
Birt Reports Use Birt Viewer to generate a report

anybox/advanced_attachment: store attachments as PostgreSQL large objects (doc).