There is a great 6 part series on customizing the user experience by Bjørn Furuknap on SharePoint magazine.

The series covers the following topics;

Part 1: Overview of the default SharePoint interface from a technical point of view

In the first article we will look at how the default SharePoint interface is built. We will look at a site, going from top-down, explore some of the the default lists, the fields used to create the basic field types, which content types are available, and how list forms are rendered.

Part 2: Modifying the default experience

This article will show you which options are available for you to modify and improve the default setup. Learn how to override the default rendering of fields or forms without voiding your supported state.

Part 3: Lists and custom list forms

The third article will cover the basics of customizing lists using different views, custom list forms, and fields.

Part 4: Content types user interface

The next article will explore how you can utilize content types to display different input forms and display forms.

Part 5: Custom fields deep dive

Ever wanted to create a new field type? SharePoint enables you to do this and it is a very powerful tool for customizing the user experience.

Part 6: Fast track to feature generation

Writing custom lists with content types by hand can take a massive amount of time. In the final installment I will share with you some tools and techniques that makes list, field, and content type generation very fast.

Bjørn also has an excellant article introducing the reader to SharePoint DelegateControl’s.