Here is the list of those bonus codes!
1/24/2012 - 2/5/2012: socialstar - 5 Stars!
11/15/2011 - 11/29/2011: farmhelp - 3 hours of machinery help
9/26/2011 - Unknown!: pretzel - Gingerbread heart for free
8/18/2011 - 18/8/2012: award - 30x super feed, 35x super fertilizer, 25x salt, 25x preserving sugar
7/20-7/24 tooltime: get 71 nails, 28 wood and a hammer
6/23-6/24 chickentogo: get five Power Feed for free
2/18-2/21 bonusfun: get 1 hour of +50% EP
2/17-2/21 droidmania: three hours use of Farm Droid
2/16-2/21 dungfun: receive 10X Super Grow
2/13-2/21 seed4free: three hours of the Farm Droid
2/12-2/21 happyharvest: three hours use of the Combine-Harvester, 1st option: harvesting
2/10-2/21 seedomatic: the Farm Droid will be available for three hours
2/10-2/21 foodmood: receive five units of Super Food
2/9-2/21 farmdroid: unlock the Farm Droid for three hours
2/7-2/21 harvesterfun: this will let you use the harvester for three hours
2/7-2/21 happyspinning: this is a code for a free spin on the wheel
Simply enter the bonus codes on the "barcode" section on the News page to redeem them. Happy farming!
Source: Game homepage