Right now, there is a massive flap occurring in the blogosphere which is known as "Mobilegeddon". Google is going to increase the relative ranking of pages which are available in both desktop and mobile themes, as opposed to only one or the other. This has got a lot of incompetent amateurs worried about whether their sites will vanish off the search landscape, but any Drupal (or other adequate CMS) user can solve this problem with a few clicks.
I had already investigated the use of a separate mobile theme for my intranet website. My home network includes virtual machines and other web-manageable services, so I have got one of those. Since I run Drupal for my public-facing site, I chose to run it behind the scenes as well. I wanted to access the site from my phone often, so I wanted a separate mobile theme to be used when I did that.
The solution to this problem is hilariously easy on Drupal 7, and here's how it works. First, you install a mobile theme. I went with Skeleton, a lightweight and responsive theme explicitly intended for mobile use. Next, you enable this theme, and then install just one module: The obviously-named Mobile Theme. This module is capable of detecting mobile browsers through all the usual means, and providing those users with another theme. After enabling the module, you should go into the theme settings and select your mobile theme. Finally, you want to move blocks around a bit; in the block settings for the theme, move stuff like the search box and login to the various footer regions.
...annnd you're done. That's it. Your Drupal site is now "Mobilegeddon"-proof, without changing the way in which it is presented to desktop users at all. Mobile users with large displays can still get your desktop site, just by asking their browser to request it.