Download the pre-release Sin Demo and sin to your hearts content.  
This demo contains 2 single player, and 2 deathmatch levels for your 
enjoyment.  Play Deathmatch over a LAN or Internet and attempt to 
pummel your friends (or enemies) into deathmatch submission.  The 
Sin Demo is a pre-release demo, and therefore we cannot guarantee 
that it is bug free, in fact we are sure that it does contain several 
bugs.  We do want your help in finding any bugs that are in this demo. 
If you find a bug, please go to http://www.activision.com/games/sin/bugs.asp
and report them.