Windows only: The more robust versions of Windows XP and Vista have built-in tools for blocking programs from certain users accounts and password-protect apps on shared accounts. Uses of the "home" versions, however, or those looking for a non-system-hacking protection scheme, should check out Empathy. The free, no-install-needed app locks up any executable you point it at, but there's a catch with the free, unregistered version—it only allows passwords of up to one character. To unlock the full-feature version, the developer asks that you send him an actual picture postcard (seriously) with your email address included. Once he receives it, he'll send along your registration. That might be too much data-sharing for the trouble, but if you just want to make people have to run through every character on your keyboard to get at an app, the free version of Empathy can make do. Empathy is a free download for Windows systems only.