From the project page:

Node linking and Metadata

The Relationship Manager module is intended to allow free-form links between nodes, recording not just that one page relates to another, but HOW it relates. ... The core module is made available to be extended by other modules that wish to make use of the generalized relationship management functions. Several examples of simple modules that leverage this power are provided as optional plugins.

The major plugins are:

From the site: "xajax is an open source PHP class library that allows you to easily create powerful, web-based, Ajax applications using HTML, CSS, JavaScript, and PHP".

There is also a fairly inactive Xajax module project at Drupal, although it may be lurking to life again, because some one did the Xajax upgrade for Drupal 5.1 recently.

From the PQuery site:

PQuery is a set of helper classes for JQuery JavaScript library.This library allows you to easily integrate AJAX , Effects and other JQuery functionality into your PHP scripts.IT has a small foot print and is compatible with PHP4 and PHP5.

Three-part "Learning PHP" series, teaching you how to use PHP through building a simple workflow application

Excellent tutorials and references.

Cook up Web sites fast with CakePHP

Adding related information and services

Bake bigger and better with CakePHP

Use Sanitize for your protection

Use CakePHP's Session and Request Handler components