If you have a pre payment meter for your gas, with british gas, then you can receive free, a card reader to plug into your computer at home.

Once you have registered with the website, it's easy to visit the website (with your card reader plugged in) enter your username and password, then insert your gas card into the card reader, using your set up debit card, put credit on your gas card.

The reader updates your gas card with the credit in seconds, which you can then insert into your pre payment meter and credit your meter.

All without leaving your house. I have been using this for two months now, it is much easier than walking across town to a garage to put the credit on.

British Gas top up website (Opens in a new window)

UPDATE:

For users of internet explorer 10 - which gets a "non compatible browser" error, British gas staff (take a bow Robert!) have provided a quick and easy work around.
Go to the website to top up, press F12, then on the top bar of the screen that appears, click on "Browser mode"
Change it to IE9 and hey presto, the website will now allow you to top up.

The reason given for the issues, is that the website is not owned by British gas, but paypoint.
British gas are currently bringing the website in house, so they will in the near future have control over it.