Using Aptana Studio for Web Development
Of late I've been using Aptana Studio for some of my experiments at home. I've been using the Community Edition, which is free, but doesn't have some of the features of the full Pro version. Overall it's a good product, but not quite to my tastes.
What is good is that it also does CSS and HTML validation as you type, creating closing tags as you open them and generally looks after markup in a sensible way. Its fairly intelligent code completion also includes support for Javascript, and with a free plugin, PHP. It plays quite nicely with libraries like Jquery too, allowing for quick insertion into new projects. It also does Ruby on Rails if that's your framework of choice.
You can flip to embedded browser previews from code view, which is nice. Then there's how it integrates with Firebug, which is quite a handy thing for doing any AJAX/JS work. Given that the project I've been working on with it uses a lot of Javascript and PHP, it seems to be quite suited to that. Actually its JSON features might have been handy too, but are only in the pro version. Similarly you're limited to plain old FTP (as opposed to SFTP) with the community version.
Aptana Studio's not as visually slick as something like Dreamweaver, and I think this is probably its biggest flaw. It certainly isn't as designer friendly, and I found the interface a bit clunky and unpolished in places. Sometimes you're just not sure what an icon is supposed to mean, something compounded by slow tooltips. Another example, its file browsing window is often slow to refresh if you drag a file into a folder outside the app itself. It also seems a bit slow in OSX, especially at starting up.
So, not quite the development environment for me, but worth a look if you need an open source space for coding webpages.