DISSECT (DIStributional SEmantics Composition Toolkit) is part of the COMPOSES (COMPositional Operations in SEmantic Space) project. It can be used to build semantic spaces from co-occurrence matrices, perform compositional operations on these semantic spaces and rely on them to measure semantic similarity between words or phrases.
To get acquainted with DISSECT, read about its main features and general philosophy in the introduction (includes licensing information), then download and set up the toolkit, and familiarize yourself with it through the hands-on tutorial.
For detailed technical information, you can use the Quick search functionality on the right (searching the codebase) and the module index links (in the navigation bars at the top and bottom right of each page).