Home surveillance systems can be incredibly expensive. If you're looking for more of a DIY approach, Instructables user Scavix shows off how to build your own small-scale system using a Raspberry Pi.
Scavix's system uses a Raspberry Pi, the Raspberry Pi camera module, some housing for that camera, and a few other smaller pieces. The end result is a home security system that can detect motion and broadcast a live stream. It's a surprisingly powerful system, all things considered, and it's cheap enough that you could set up a few of them if you like. If you're looking for something similar that's a bit easier to set up, we're fans of Ninja Blocks.
Raspberry PI as low-cost HD surveillance camera [Instructables]