LibCheck allows you to compare two versions of an assembly, and determine the differences. The tool reports the differences as a combination of 'removed' and 'added' APIs. The tool is limited to looking only at APIs (i.e., it can't check for behavioral changes), and only compares public differences, or changes which are deemed to be 'breaking.' The tool can be used to quickly determine what has changed between one version of your assembly and another, and can help ensure that you won't introduce any breaking changes to clients of your assembly. Instructions and intended use of the tool are described in the 'libcheck tool specification' document with the zip file.
Download it here.