7-Zip Archiver
I love finding free tools that work better than commercial versions. I have always tried to keep a pair of file archiving utilities on all my PCs: WinZip and WinRAR. However, I was reading some comments where people were talking about WinRAR, and someone mentioned 7-Zip. I quickly discovered that it's a SourceForge-hosted open-source project that handles more file formats than WinZip and WinRAR put together, integrates into the Explorer shell, and is just as easy to use. I've already deleted WinRAR and plan to use this one almost exclusively.