Latest tested version: Z3 4.0
ftp://ftp.research.microsoft.com/downloads/0a7db466-c2d7-4c51-8246-07e25900c7e7/z3-4.0.msi

Windows: http://research.microsoft.com/en-us/downloads/0a7db466-c2d7-4c51-8246-07e25900c7e7/
Other platforms see: http://research.microsoft.com/en-us/um/redmond/projects/z3/download.html