Gravit (Popularity: )
http://gravit.slowchop.com Gravit is a gravity simulator which runs under Linux, Windows and Mac OS X. It's released under the GNU General Public License which makes it free. It uses Newtonian physics ...
NEMO (Popularity: )
http://bima.astro.umd.edu/nemo/ [open source - Unix] An extendible stellar dynamics toolbox; has various programs to create, integrate, analyze and visualize N-body and SPH-like systems. In addition there are various tools to operate ...