Downloading OSEM 6.1 from the developer's website was possible when we last checked. We cannot confirm if there is a free download of this software available. This software was originally created by OSEM Technology. The following versions: 6.1 and 1.4 are the most frequently downloaded ones by the program users. We recommend checking the downloaded files with any free antivirus. The program lies within Education Tools, more precisely Science Tools.
From the developer:
OSEM is designed to reconstruct projection/sinogram sets in Interfile format. It is a command line driven program compilable on many hardware platforms, running a variety of operating systems. Executables for Sun and WindowsNT systems are available. It is also known to work on DEC and SGI machines.
You may want to check out more software, such as Maxima and Minima, DAC Express or IUCLID5, which might be similar to OSEM.