If you haven't already heard of S.W.I.F.T., here is the basic idea:
To create interactive web content, there are a few options: Flash, Java, Silverlight or HTML5. Java has problems with the web: any java applet comes up in a window marked as 'unsafe', and it doesn't work very well for anything graphics-intensive. Silverlight is Windows-only. That leaves Flash and HTML5. There are two ways to create Flash content: in a GUI (Adobe Flash) and by hand, as text (Adobe Flex, swfc, ming etc.). In HTML5, you only have the latter option. Adobe Flash is a very good tool for this, with two problems: one, that it won't run on Linux; and two, that it costs $700.
Lightningbeam aims to address this, by providing a free and open-source editor for dynamic web content. As some platforms support Flash but not HTML5 (e.g. Internet Explorer), and others support HTML5 but not Flash (e.g. the iPhone), Lightningbeam allows you to seamlessly create both from a single project.
If you have heard of S.W.I.F.T., here is what is new:
Lightningbeam is a complete rewrite, based on lessons learned from SWIFT. It runs on other platforms than Linux, without installation. The HTML5 "engine" has undergone vast improvements in performance (running nearly 100 times as fast in FireFox). The underlying system is now much cleaner, making it easier for Python plugins to run.
Lightningbeam is still under heavy development: it is incomplete, but many new features are being added every day. To see the latest features added, visit https://github.com/skykooler/Lightningbeam.