Puppy Linux is a distro I keep coming back to. No matter how entrenched I become with any flavor of Ubuntu -- sans the Unity desktop -- or Linux Mint's Cinnamon and KDE desktops, nothing can beat the speed, convenience and reliability of Puppy Linux on a stick.
Boot it directly from a hard drive or from an external drive; either method is fast and convenient with no dual boot menus required. Puppy barks impressively. The credit for that is its design to run in RAM rather than pull its code from slower physical or optical drives.
Head developer Barry Kauler released version 5.4.3, aka Precise Puppy, in Oct. 2012. This latest release makes Puppy Linux better than ever. It is a fully functional Linux OS that puts its power and effectiveness in performance, rather than gobbling up resources with excessive eye candy.
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