Pretty much anything that you download via a DMG that has the "drag to /Applications" installation method can be put anywhere on your hard drive (although do bear in mind that other users may not be able to see their installation if placed in a folder that you "own").
Any applications that use an installer (say, "Photoshop" or "Quark XPress") should not be moved from the installation location selected through the installer (some will let you change the installation location -- that's fine), as it will likely break updates, plug-ins, etc.
Any applications installed on the Mac at purchase, installed through a system update or via an Apple installation disc should not be moved from the default location -- it will break Software Update and will likely cause other issues.
As it is, why would you want to keep programs anywhere other than /Applications? It's handy, you can get to it from the keyboard ([CMD]-[Shift]-[A]) and it's viewable by all users. If you need quick access, you can use an Alias, the Finder Sidebar, the Dock and/or the Finder Toolbar. (Not to mention very customizable launchers like Quicksilver and LaunchBar (my personal fave).)