Dojo is a cross-browser AJAX/DHTML toolkit for building dynamic Web UIs. Dojo includes a rich-text editing component, an aspect-oriented event system, rich drag-and-drop support, a rapid-prototyping framework for building new widgets, and a full-featured abstraction for XmlHTTP I/O. In short, it's a very fast way to build Web interfaces.
capictcha generates CAPTCHA images from arbitrary strings. It supports the use of any TrueType/OpenType font and an alternate (more stable) font for punctuation and other characters, and allows you to specify the size and output text, JPG or PNG graphic output, and MD5 lookups of values you don't want to expose as plain text (which could be used to give a simple text table relating email addresses to their hex MD5 hash). Individual character tweaks, overstriking, and random lines are easily configurable.
r80v5 is a fast, lightweight HTTP server written in REBOL, supporting the execution of server-side REBOL code embedded in HTML files. It is easily extended by mapping modules to a resource pattern. For example, one module could be called for all *.txt files, but a different module could be used for all *.txt files in the directory 'source/'.
Vanilla is a WikiClone written in REBOL. It is useful for collaboratively working on projects, tracking todo items, managing adresses, and creating Web logs. All page editing takes place in the browser. Vanilla features a simple templating system, a full-featured user and session management system, and an extension architecture for writing simple Web applications in REBOL.