What are the available tools/tutorials for learning to develop ExpressionEngine addons (modules, fields, extensions, plugins and accessories)?
The following list should get you started:
ExpressionEngine Add-On Development Docs: links to guidelines, reference material, and tutorials for developing Plugins, Modules, and Extensions for ExpressionEngine
ExpressionEngine Add-on Development Recipes: a collection of articles and tutorials to help you learn how to build ExpressionEngine add-ons
Add-On Development Cheat Sheet: freely downloadable cheat sheet and Accessory for ExpressionEngine 2.x. Its purpose is to help add-on developers get used to coding for EE 2, speed up development time and increase productivity.
Building an ExpressionEngine Add-on: Commercial yes, but an excellent video by well-respected add-on developer @low giving a solid introduction into planning and building an add-on.
Pkg.io: Package Builder for ExpressionEngine which will quickly generate boilerplate code for all types of ExpressionEngine add-ons. It includes helpful comments in the source and will jump-start development of your custom add-on.
DevKit for EE 2.0 Generate a new module skeleton, yaml 2 dbforge code generation and global variables / snippets sync from filesystem.
Channel-Data (objectivehtml) A helper class that compiles all the necessary models to easily gather ExpressionEngine channel data. The main benefit of this library is its ability to be dropped into any existing add-on and retrieve channel data extremely easily without having to load all the models.
EE-Add-On-Builder SublimeText plugin to easily build ExpressionEngine Add-Ons.
Open API - Front-end API for ExpressionEngine that provides authentication and CRUD functionality.