PureSim 4 - Lite Version
The latest FREE (PureSim Lite) version can always be downloaded from this page. The Lite version requires that you play in "PureSim Challenge" mode. If you get below a certain "GM Score" in challenge mode, you risk getting fired from the association, and you'll have to start over. The better GM you are, the longer you get to play for free 
If you get canned, and are attached to your association, you can always upgrade to the full version and get another few seasons to see if you can turn things around. The full version also has tons of other modes available.
230 New Features and Tweaks for 2011