In my opinion SSAP2010 is an interesting freeware to verify slope stability and despite of other commercial softwares, it considers some inhomogeneities for example: soils, groundwater at different levels, different surfaces and stability elements (wall, sheet pile, etc.).
SSAP means Slope Stability Analysis Program and some days ago was updated to 4.2.0 version, you can download it at the website of the author: Dr. Lorenzo Borselli directly from here. I suggest you to read the technical manual, it is useful to understand how the program works.
I put the full article with an example only in the Italian section, because the software is completely in Italian and it isn't easy to use without a deeply reading of the technical manual.