7-Zip

Windows can open a zip, but the moment someone sends a 7z, a tar or a split archive you need a real archiver. 7-Zip is it: small, mostly LGPL-licensed, free to use, and with no trial nag to click through. Its own 7z format, using LZMA and LZMA2, squeezes files harder than zip, and even when you stick to zip, the project reports ratios 2 to 10 percent better than PKZip and WinZip. The right-click menu entries do most of the work, so the plain, old-fashioned main window rarely needs opening. Check for new versions now and then, because security fixes only reach you when you install them.