7-Zip

7-Zip

If you frequently deal with zipped archives (and not just the .ZIP kind), then you need something a little more powerful than the usual WinZip that everyone uses. 

For starters, 7-zip is open-source and completely free, it can handle packing/unpacking of every format out there, it integrates neatly with the system (appears in right click) and also has its own high-compression format called 7z.