iUnarchive is the best application to zip and unzip in Mac and it now comes to iPhone.
iUnarchive Lite is capable of decompressing zip, rar, 7-zip, tar, gzip, bz2/BZip2, LhA and StuffIt.
Furthermore, you install this application on your iPhone and it appears in the emerging menus in programs like Dropbox, Gmail, Mail or Safari, so if you find a compressed file, you will be able to click on it and choose to open it with iUnarchive.
Like this, we can resolve one of the most annoying problems we had using iPhone to open attachments that are usually compressed.
If you have an iPad, you must use the complete version.