jQuery UI provides abstractions for low-level interaction and animation, advanced effects and high-level, themeable widgets, built on top of the jQuery JavaScript Library, that you can use to build highly interactive web applications.
Mojikan is a new site that aims to introduce a new concept of interactive tools for online social behavior. They are still heavily involved with their site development, but you can still explore parts of their site, and see what they plan to offer.
Kig is a program for exploring geometric constructions.
It is meant as a better replacement for such free programs as KGeo, KSeg and Dr. Geo and commercial programs like Cabri.
Eukleides is a Euclidean geometry drawing language. Two softwares are related to it. First, eukleides, a compiler which allows to typeset geometric figures within a (La)TeX document. This program is also useful to convert such figures in EPS format or in various other vector graphic formats. Second, xeukleides, a GUI front-end which makes possible to create interactive geometric figures. This program is also useful to edit and tune some Eukleides code.