Number 5 on "My Top 10 Most Useful Free Programs" is IZArc (Available at http://www.izarc.org/)
From their website "IZArc is the ultimate freeware archive utility supporting many archive formats like: 7-ZIP, A, ACE, ARC, ARJ, B64, BH, BIN, BZ2, BZA, C2D, CAB, CDI, CPIO, DEB, ENC, GCA, GZ, GZA, HA, IMG, ISO, JAR, LHA, LIB, LZH, MDF, MBF, MIM, NRG, PAK, PDI, PK3, RAR, RPM, TAR, TAZ, TBZ, TGZ, TZ, UUE, WAR, XXE, YZ1, Z, ZIP, ZOO"
I have found this app to be better than winzip or the built in compression tools in XP and Vista. I have not come across a compressed file that it has not been able to open yet. It also has the ability to encrypt and decrypt files.
The interface is extremely simple and easy to use with drag and drop functionality.
To create a new archive you can simple drag the files onto the apps window and click the Add button. IZarc will automatically create the zipped file for you.
To open an existing compressed file you simple double click on it.
IZArc also adds "right click" functionality to the windows explorer to make if easy to extract files from within compressed archives.
There is also a portable version available that will run from removable media such as USB keys and CDroms. Have a look at this version here http://www.izarc.org/izarc2go.html