Smart Home Hacks covers Windows, Mac, and to a lesser degree, Linux systems. For a more detailed overview of the contents, and some samples, see the O'Reilly catalog at: http://www.oreilly.com/catalog/smarthomehks/

Many of the techniques described in the book don't require a computer at all, by the way. But one important point to keep in mind about home automation is that it's about 80% technique -- the details of how to implement it using a specific platform or application are pretty easy to translate from one system to another, so even if a particular technique is described using software that you don't have it's usually not to much trouble to re-implement it. Besides, in most cases, you'll want to do that anyway so you can customize it for your own particular desires.

I use a 2nd-generation iBook, KeyLime in color. For the purposes of home automation, it works well because of the battery back-up and silent operation.