One of the beauties of modern computing is that we can buy these fairly cheap machines–or even just pick "obsolete" computers out of the bins–and get them to do anything we want them to.
For example, here’s an article about Lightweight Home Security with Indigo and Asterisk. Now, this is not all about Linux, but it is about brewing up your own application with (some) open source software.
Scott Laird was looking for a way to secure his home from his 4-year-old daughter (to keep her from sneaking out) but couldn’t find anything that suited for a reasonable price. So he decided to cook up his own solution: hook up a Mac to home automation hardware sensors, and have that system feed into Asterisk, which in turn forwards information about open doors (through which a wayward toddler can exit the house) to the household phone system.
Maybe not something you can do (or would want to do), but it gives you an idea of the kind of benefits open source offers. And anything that’s doable–but not easily doable–becomes a business opportunity to the person who knows how to do it.