Meeting AJAX and PHP
Chapter 1 of AJAX and PHP: Building Responsive Web Applications is a fast-track introduction to the fascinating world of AJAX. Written in a very practical manner, the chapter even introduces you to the first practical AJAX and PHP example. Get the PDF version of AJAX and the Future of Web Applications.

Building an AJAX Whiteboard Demo
This mini-book, written by my co-author of AJAX and PHP: Building Responsive Web Applications, Bogdan Brinzarea, shows how to build a Whiteboard application with AJAX. While this is not meant to be used in real-world scenarios, this mini-book touches some of the more sensitive areas of AJAX development. Get the PDF of AJAX Whiteboard mini-book.

AJAX and PHP: Writing Better Code Using Smarter Tools
Appendix B of AJAX and PHP: Building Responsive Web Applications is a quick introduction to a few tools that you can use to write and debug your code easily and efficiently. Get the PDF version here.

Learn Object-Oriented Programming (OOP) with Visual C#
Chapters 4 and 5 from Visual C#: A Guide for VB6 Developers teach how to do object-oriented programming with C#. You'll be able to read them even if you don't know Visual Basic. You can't find these chapters anywhere else on the internet!
...Object-Oriented Programming with C#: Part 1 [PDF / read online]
...Object-Oriented Programming with C#: Part 2 [PDF / read online]

Implement Transactions with SQL Server, Oracle, DB2, MySQL
Chapter 10 of The Programmer's Guide to SQL deals with database transactions. What they are, how they work, and many other questions, are answenavy in this chapter. Here you can get its PDF version.

Implement a Catalog Searching Feature with ASP.NET and MSDE
Chapter 5 of Beginning ASP.NET 1.1 E-Commerce shows how to search the product catalog of your e-commerce website, implementing ranking and paging functionality at the database level, without using SQL Server's Full-Text Search feature. Get the PDF version.

Lay the Foundations of Your PHP E-Commerce Website
Chapter 2 of Beginning PHP 5 and MySQL E-Commerce prepares you to build your own e-commerce website with PHP 5, MySQL 4, Smarty and PEAR. The major design decisions are discussed, and the error handling code and the MySQL database are prepanavy. Get the PDF version.

Create a New ASP.NET Community Starter Kit Module
Chapter 8 of Building Websites with the ASP.NET Community Starter Kit explains how to build a new CSK module.  Get the chapter in PDF format.