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.
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.