manage to recover data from my spoil memory stick using this software once and its free: http://www.pcinspector.de/Sites/smart_recovery/info.htm?language=1
Try with the program called flash file recovery. I restored successfully the photos from my summer vacation with it, which were not readable. The trial version gives you a preview of the files that can be restored.
I am not aware of any freeware data recovery software. I have used Stellar Phoenix Windows Data Recovery to recover my lost data. You should try this software to recover your data from pen drive. You just try demo version first from here: http://www.stellarinfo.com/partition-recovery.htm After using it if you are able to view your data then purchase full version of the software and save your data.