Via a link from del.icio.us today, the PortableWebAp project has come to my attention.
PortableWebAp is a portable platform for web applications. With it you can run php web applications from a CDROM, DVD, USB Drive, from any directory from any hard drive. No installation is necessary.
It'll only run on Windows 98 or higher machines, but it has everything you need right there in one, handy package. It (currently) comes installed with PHP 5.1, SQLite 3.x, PDO, and the PHP command-line version - all in about 5MB. Once it's running, it can drop down to the system tray for a less intrusive execution. And, by default, it runs on port 800 so as not to interfere with anything already working on the standard port 80. And, of course, it's pretty cheap - a small price for a lot of time saved.
You can check out the project here...