How to install a QGIS plugin when offline?

Due to various IT policies at my workplace, QGIS is installed on a machine that is not connected to the internet. I wish to install a couple of QGIS plugins on this system.

I have downloaded the required plugins from http://pyqgis.org/repo/contributed. How do I install them in QGIS?

Answer

You can just extract them into the .qgis/python/plugins folder in your home directory.
If you are using QGIS 1.9.0. (available as nightly build) you need to extract the archive into .qgis2/python/plugins instead.

The folder structure should look like this:

.qgis
├── python
│   └── plugins

│        └──plugin folder
│        └──plugin folder
│        └──plugin folder

For example this is an extract of what mine looks like:

QGIS 3 Note: When QGIS 3 is released it will contain a "Install from Zip" menu item to remove the need for you to manually do it.