I will agree that Visual C# Express is a great tool - it's the reason I went with C# (plus already knowing C++ :) Think about whether you should use C# instead of JS.
But if you definitely want a Javascript editor with auto-completion, there's one that is very good, Eclipse. There are some drawbacks - you must download/install a plugin to have it work with Javascript (at least if you want auto-completion, you can edit anything). Second, Eclipse is a big, slow program (that requires the Java 1.5/1.6). It might be overkill for simple scripting.
Eclipse
Javascript Plugin for Eclipse
I've never used this plugin, so I don't vouch for it (I use Eclipse for Python).
Update: If you haven't decided on a language (or might change), here's a question discussing the pros and cons of each:
JS versus C# versus Boo
Update 2: I just went to the plugin site, out of curiosity :) and discovered that Adobe Labs now owns the plugin. If you go to the suggested Adobe download site, you need to have an account to get access. You do not have to create an account to get this plugin!
What you can do - Eclipse has the ability to auto-install (or update) its plugins. You give it a URL, and it will get the plugin itself. How you do it: first, you need to know the URL. In this case, if you look at the right sidebar, you'll see an Installation/Update page. Step 6 has the URL needed to auto-install. Then, in Eclipse, you go to Help->Software Update and the Available Software tab. Then click Add Site, and give it the URL you just copied. Then check the box by JSEclipse, hit Install, and away it goes...
Update 3: Why you should reconsider C#:
Okay, you've probably already installed Eclipse. :) But still... I was thinking about the word auto-complete, and realized it doesn't cover everything. Eclipse with a Javascript plugin will auto-complete Javascript keywords - but not Unity objects. There is a difference.
On the other hand - Unity is integrated with Visual Studio tools, and once you configure it, will auto-complete Unity objects. So if you type ga, you'll have GameObject as an option to complete the word. In addition to tooltip help bubbles for Unity objects, including parameters.
So if that's the kind of behavior you are thinking about for the word auto-complete - I know Visual C# Express will do it. I have no idea if Eclipse can be made to do it, but have my doubts.