One of the highlights (for me) at the Silicon Valley Ruby Conference was Alex Jaffee's presentation on the powerful client/server-style JavaScript and Ajax tools his company Pivotal uses.
Alex mentioned that they do all of their JavaScript work in Firefox with the Venkman JavaScript debugger, the FireBug DOM and variable inspector, and the Web Developer toolbar. I've just been working on a JavaScript-heavy project and thought I'd check them out.
Wow.
For someone who has been using the time-honoured 'alert messages and prayer' methodology since 1995, it's like turning on a floodlight when before now I've been striking matches.
In the past, I've tried to limit my JavaScript use to validations, popup windows, and the occasional special effect, but I've been working on a project that had some pretty heavy JavaScript requirements, including 'faking' a database layer. I don't think I would have been able to do it and keep my sanity without these tools.
One of Alex Jaffee's points in his talk was that the web client is becoming Thick again, and although DHTML, CSS, and JavaScript are all kind of half-complete and messy to work with, the next round of innovation is likely to come through better interactions on the client side and you'll need to use these languages a lot more than before.
If you do web development, you should check them out.