From ZRT Labs:
The zip file format offers much in terms of data storage and network transfer. Its potential weakness, however, lies in the fragility of the file structure. If for any reason some part of a zip file becomes damaged, standard zip tools will be unable to extract it. The cyclic redundancy check makes file extraction impossible if the CRC values of the original file and the extracted file donÃ??Ã?Â¢Ã?Â¢?Ã?Â¬Ã?Â¢?Ã?Â¢t match. Zip Repair Tool provides the user with a cutting edge technology that helps repair zip structure and extract zipped files. Full automation of the zip repair process and a user-friendly interface make zip repair a snap. After the user locates zip files, the program scans each file to assess the damage. Then Zip Repair Tool deploys its robust engine to recover zip files wherever possible. High speed and accuracy are the distinguishing features of the technology. The zip repair engine can fix zip files of any size and number. Big files can exceed 4 Gb and can "span" multiple disks, but with Zip Repair Tool you can easily recover zip files of any size and number of volumes. The batch mode offers an additional benefit of automating zip repair on multiple files.
First of all, it worked perfectly, recovering my un-openable zip files. It was extremely quick and easy to use. The demo version seems to be fully functional, though I didn't test the 5mb file limit -- my corrupt files were a bit smaller.
Minor nit -- the only feedback from its work is a long and detailed log file. My small brain would like, in addition, a summary status, with information on recoverability of each file within the zip.