The most common tool kit for Augmented Reality Applicatons is known simply by the name ARToolkit and it contains a library of applications for viewpoint tracking and virtual object interaction. It works by understanding the camera's actual position in relation to objects so it can render graphics over images. Some of the top features of ARtoolkit are that it is free and open source, fast enough for realtime applications, and it has the ability to understand its location with a single camera. The homepage for the ARtoolkit can be found here.
ARToolkit related downloads can be found at SourceForge, since the original has not been updated for a few years but several of the authoring tools have been upgraded and expanded since that time. Augmented reality toolkits may be instrumental in making your own AR apps that can be sold in online app stores, or just shared among likeminded friends and homebrew software enthusiasts.