In most distributions that come with KDE Software Compilation 4.3 and above, opening Dolphin for browsing the filesystem will give you Preview Mode Button. We will expand on this option.

On the right is the Information Panel, which in addition to showing info about the file (type, size, modified etc.), will allow you to preview the majority of files (Text, Image, PDF, Open Document). This happens after you select a file or hover your mouse over it.

What’s more, the Information Panel enables playback of Video and Music files. A small media player will appear, with just a play/stop button and a bar, but it’s more than enough.

You can also add an extra information popup window that shows a preview and some information