At the top of the current heap of free web design software we find Aptana Studio 1.2.1.
Created by a company with commercial interests, it combines the slickness of an off-the-shelf tool with the benefits of open source.
Aimed at Web 2.0 coders, Aptana handles web scripting easily, with features for advanced application development such as Ruby on Rails, Adobe AIR and iPhone support through free plug-ins.
Based on the open-source Eclipse IDE, Aptana comes in both commercial and 'community' versions. It's the latter that we recommend.
Eclipse is well-known in the open source community as a tool to be trusted. Adobe uses it as the foundation of Flex Builder – the tool used to develop server side Flash applications.
With plug-in support for every scripting language you'll ever need, Aptana Studio is the clear choice for coders. If it had a visual HTML design window, we'd give it 6 out of 5.