Subversion & Source Control
Whilst this is a new site, I have been running a site for my photography for a number of years. Recently, as part of the build for this site, I decided I should upgrade the core system the photography site runs on to Drupal 7. For various reasons this went wrong. Whilst I had a backup, it was incomplete for several reason. For a time, I felt that I had lost all the content for the site. This was naturally concerning and it made me realise that I had not made any effort for source control. As it was a pretty basc HTML and PHP site, with a handful of megabytes in a MySQL database I'd been working on essentially live code.
Having recovered the site, I've implemented a local Subversion Repository. By managing source code locally, as you would in a commercial environment, you ensure you have an ongoing roll-back strategy.
The number of companies which don't use a source code management system, despite the low cost and low effort is quite remarkable.