Applicability Domain Toolbox
The Applicability Domain toolbox for MATLAB is a collection of MATLAB modules for calculating the Applicability Domain (AD) of a model for a set of test samples and verify if they fall inside or outside the model AD. For a test sample outside the AD, the model shouldn't be applicable since the prediction reliability will be low.
The toolbox implements a set of AD approaches based on several strategies, such as: bounding box, bounding box on PCs, convex hull, leverage, distance to centroid, k Nearest Neighbours (kNN) approach with fixed k, k Nearest Neighbours (kNN) approach with variable k, Probability Density Function-based methods. A graphical user interface (GUI), which allows an easy AD evaluation and analysis of results, is also provided.
The toolbox is freeware and may be used (but not modified) if proper reference is given to the authors. Preferably refer to the followign papers:

F. Sahigara, D. Ballabio, R. Todeschini, V. Consonni (2013). Assessing the validity of QSARs for ready biodegradability of chemicals: An Applicability Domain perspective. Current Computer-Aided Drug Design, 10, 137-147

F. Sahigara, K. Mansouri, D. Ballabio, A. Mauri, V. Consonni, R. Todeschini, (2012) Comparison of different approaches to define the Applicability Domain of QSAR models, Molecules, 17, 4791-4810

You can freely download the MATLAB modules:

download the Applicability Domain Toolbox