DroidDraw seems to be very useful. It has a clean and easy interface and it is a freeware. Available for Windows, Linux and Mac OS X. I advice a donation.
If you don't like it, you should take a look at this site. There are some other options and other useful tools.
Allow me to be the one to slap a little reality onto this topic. There is no good GUI tool for working with Android. If you're coming from a native application GUI environment like, say, Delphi, you're going to be sadly disappointed in the user experience with the ADK editor and DroidDraw. I've tried several times to work with DroidDraw in a productive way, and I always go back to rolling the XML by hand.
The ADK is a good starting point, but it's not easy to use. Positioning components within layouts is a nightmare. DroidDraw looks like it would be fantastic, but I can't even open existing, functional XML layouts with it. It somehow loses half of the layout and can't pull in the images that I've specified for buttons, backgrounds, etc.
The stark reality is that the Android developer space is in sore need of a flexible, easy-to-use, robust GUI development tool similar to those used for .NET and Delphi development.
Not saying this is the best way to go, but its good to have options. Necessitas is a project that ports Qt to android. It is still in its early stages and lacking full features, but for those who know Qt and don't wanna bother with the terrible lack of good tools for Android UI would be wise to at least consider using this.
Creating an App Inventor app begins in your browser, where you design
how the app will look. Then, like fitting together puzzle pieces, you
set your app's behavior. All the while, through a live connection
between your computer and your phone, your app appears on your phone.
I use it in real applications, not only for fast prototyping or dialogs and its well tested over the years. The concept is based on the model view control principle and for most common scenarios there are ready to use components which automatically look correct on any device. I don't say it should be used for any UI (e.g. listviews should be done by hand) but for most usecases this should be quite handy ;) Oh and feel free to fork it and improve it further if you want
This is an old question, that unfortunately even several years on doesn't have a good solution. I've just ported an app from iOS (Obj C) to Android. The biggest problem was not the back end code (for many/most folks, if you can code in Obj C you can code in Java) but porting the native interfaces. What Todd said above, UI layout is still a complete pain. In my experience, the fastest wat to develop a reliable UI that supports multiple formats etc is in good 'ol HTML.