Oracle Application Express (Oracle APEX) is a rapid web application development tool which uses the Oracle Database.

It allows the development of professional, fast, secure and scalable web-based applications. Using just a browser it is possible to create applications with limited programming experience.

APEX is a no-cost system for licensed users of Oracles database technology.

The Oracle official website includes downloads, tutorials and resources
Oracle Official Website

The Oracle APEX forum is the home to a community of developers where questions can be asked and answered
APEX Forum

Blogs
The following blogs contain a gold mine of hints, tips and tutorials for Application Express. Often this is information you won't find in a book or on a training course :

Denes Kubicek ApEx BLOG
http://deneskubicek.blogspot.com/

Yet Another Blog About Oracle Application Express
http://joelkallman.blogspot.com/

apex2rule-the-world
http://apex2rule-the-world.blogspot.com/

Documentation
The official document library contains the set of documents relating to a specific version of APEX. This includes Release Notes, Installation Guides, Express Developer's Guides, Administration Guides and API References.

APEX v4.1 Documentation Library

APEX v4.0 Documentation Library

APEX v3.2 Documentation Library