Earlier a free version of the application has been available, but added advertisements into the application. Now instead the development tool itself is available for free, and instead "PRO" and "PREMIUM" support tiers are available for those that need it.
While this will also be possible with the latest Flash Builder 4.5 and Flex SDK 4.5, Flash Builder and OpenPlug have different ways of running the code on the target device.
While Adobe's answer to creating applications that run across platforms is to bring their Flash and AIR runtimes to each platform they plan to support, OpenPlug goes another way. OpenPlug takes the code you write in ActionScript3 and MXML and compiles it for the platform you want to run your code on, producing a native application for each platform.
OpenPlug extends the APIs available in Flex adding support for more mobile APIs with each new release. With OpenPlug native applications can be developed for the following platforms, iOS, Windows Mobile, Android, and Symbian. Since it uses a modified Flex SDK, it is possible for developers to use some of their existing Flex application code (the core bits, not the UI), as a base for developing a mobile version. OpenPlug applications can also be extended using native code components. So one can write native code in C, Objective-C, or Java that can gain better access to the systems, allowing existing code to be reused without losing the unique advantage each platform allows.
OpenPlug itself is an Eclipse-based IDE that runs on Windows and Mac OSX, like the Flash Builder IDE, and can in fact even be installed as a plug-in to Flash Builder rather than a standalone application.
OpenPlug has a number of advantages over Flash Builder for those wanting to create mobile applications using Flex code, such as being free, supporting native UI components, native extensions and not requiring any additional runtimes.
You can find out more about OpenPlug on their site, and can download a copy after registering for a free account on their site.