Linux: If the graphical demands of previously mentioned Linux launchers GNOME-Do or AWN are too much for your needs, Kupfer might be a perfect fit. It works in a similar fashion, but uses only spare resources to do so.
Kupfer doesn't require compositing abilities from your Linux desktop, so any graphics card and most any GNOME-based desktop can get it up and running. Its basic functionality is similar, though—hit your Kupfer shortcut (Ctrl+Space by default), type the first few letters of a file, folder, or application you're looking to run or open, and Kupfer opens a two-panel window. The left-hand pane shows a result, while the right-hand spot lets run, open, delete, "favorite" or otherwise manipulate the file. Like AWN and GNOME-Do, Kupfer is powered by plugins, which roll in as developers catch on.
Kupfer requires a semi-manual install, detailed at the Lazy Ubuntu via link. It asks for, and requests, an uncommon Python package called keybindings, but a link explaining how to manually set a keyboard shortcut for Kupfer can be found at Kupfer's main page. It's a free download for Linux systems only.