OS X currently has a bug wherein if you double-click a file to unzip it, the unzipping process can get jammed without the files being extracted. Even if you Force Quit the unarchiving app, the problem will reappear next time you try and unzip.
Apparently, the following command typed into a Terminal window (Terminal lives in the Utilities folder of the Applications list of Finder) will free things up — ensure you save your files in all open apps before using it:
sudo killall -KILL appleeventsd
Unfortunately I’m not able to test this because, although I’ve suffered from the issue in the past, I’m no longer able to replicate the problem. Let me know how you’re getting on in the comments below.
Another solution, and perhaps a better one, is to install The Unarchiver, which handles decompressing of all kinds of archive files.
Lets hope this bug is fixed in the 10.8.3 update that’s due to hit soon. It was first spotted in October last year.