PhoneGap Build is currently in beta and to gain access you have to register your details and wait patiently, however I found I got my beta invite within a few days. It builds apps developed using the open source mobile framework PhoneGap entirely online so you don't have to worry about installing SDKs or development environments on your computer. There are APIs to interact with phone features like the GPS, sensors, camera and file system and the theory is that you write the code once and it will run on lots of different devices. You can even incorporate JQuery Mobile to make the app look really good.
Once the app is finished and packaged in a particular way (instructions on the PhoneGap website), it can be uploaded to the PhoneGap Build service. This builds for five platforms, but you might have to do a little work to get it to build for all five successfully. Four of these platforms are easy to set up, but one is a bit more difficult. Can you think of which one? Yes, that's right iOS! Surprised? To get the build service working for this you will need an Apple Developer account and a Mac to set up a profile, details here: https://build.phonegap.com/docs/ios-builds.
To get the Android app that was built into the Market you will need to sign the app, details here: https://build.phonegap.com/docs/android-signing. However if you just want to test the app you can miss this step put as it can be installed straight onto a phone. The other platforms, Palm webOS, Symbian and Blackberry don't seem to need any special steps. However the build for Blackberry can be temperamental and some of the issues are listed at: https://gist.github.com/778233.
The actual build only seems to take a few minutes and then you are presented with links to the newly built apps (if successful). For some platforms QR codes are provided that you can use with your mobile to get the app straight on there. When you use them though you might have to sign in on your phone to your PhoneGap Build account in order to get the app to actually download.
It is interesting to see mobile phone development start to drift away from a traditional developer environment on a computer and into the cloud, but PhoneGap Build seems to offer a promise of removing a lot of the complexity involved with targeting multiple platforms. It might not be suitable for all applications (you wouldn’t want to use this for many games for example) but for many cases it will be good enough to build a decent application and reach a large number of people quickly. I've only just started experimenting with the platform, but it feels like it is an approach with a lot of potential.