First is Metaio Unifeye Design. Available freely in an (almost fully featured) demo version, this tool is far from being a toy. You can create elaborate scenarios of marker based and marker-less image based augmented reality without writing a single line of code. You can test your creations with your web-camera or using a prerecorded video clip of a marker. There's even a tool for creating new markers.
However, such flexibility does come with a price. I had a hard time working with the user interface beyond the basic functions, and I'm quite sure I only scratched the surface of what's possible with this tool. That's why I had this application installed for about a month now, but I was hesitant about writing anything about it.
Metaio claims that video tutorials are coming shortly, and I think they are very much needed (and I'm a programmer in my spare time :).
In more somber news, you can't use your own 3d models, or export your scenarios as a stand alone application. You probably have to pay for those features.
Now, if Metaio Unifeye Design was a bit too complex for you, the next application is truly augmented reality for beginners. It's called Atomic, and it wraps ARToolKit to give you the basic functionality of augmenting markers with VRML 3d objects when you examine them using your webcam. The application source is available to download as well, so eager programmers may add more features to it in the future. Another advantage of this tool is that it allows you to import and use your own models. However, don't expect to amaze any AR veteran using it.
Atomic (Atomic via Rising Wisely)