FRAGSTATS: Spatial Pattern Analysis Program
"FRAGSTATS is a computer software program designed to compute a wide variety of landscape metrics for categorical map patterns.The original software (version 2) was released in the public domain during 1995 in association with the publication of USDA Forest Service GTR PNW-351." [from website]
This link provides access to the current public domain version of FRAGSTATS, the FRAGSTATS Listserve, documentation, and other related links.
McGarigal, K., and B. J. Marks. 1995. FRAGSTATS: spatial pattern analysis program for quantifying landscape structure.
Landscape Ecology, GIS/Mapping