AssetCentre, if you've got the bank, is great for this. You automatically get a new revision created with timestamp when a edited file is checked back into the library. All user activity is automatically logged on the server, and can show exact neutral text of rung edits, tag values changed, controller properties changed, etc. This info is sortable by asset and user so tracking is very easy.
There's also a cool disaster recovery feature that can do scheduled backup of running plc programs and compare of archive files versus running programs for unauthorized changes.