Ajax Framework

ZK is an open source AJAX framework that assists in rapid development of rich Internet applications.

XML based CMS

GetSimple is an easy to learn, open source CMS that uses XML for storing data.

MegaZine

Megazine 3 is an open source Flash object that generates an interactive book interface via external XML.

TiddlyWiki

TiddlyWiki is a single html file which has all the characteristics of a wiki – including all of the content, the functionality (including editing, saving, tagging and searching) and the style sheet.

Still recreating the wheel

Jeffrey Zeldman discusses the future of open source.

Cushy CMS

CushyCMS is a free, easy-to-use Content Management System that allows you to expose editable regions inside your html document.

Open Source Photoshop Brush Viewer

Manage and preview your Photoshop brush library with ABRviewer.

Opentape

Opentape is a free, open-source package that lets you make and host your own mixtapes on the web.

Build Offline Web Applications

Roadsend compiles your PHP into stand-alone executables that do not require an interpereter.

Galleria

David Hellsing has put a nice twist on this AJAX Thumbnail Gallery using Jquery and some smart preloading. Take if for a test drive and pick up the latest source from Google Code.