I recently had the privilege of building a computer at my place of employment, using the following parts:
The result is an inexpensive, fan-less computer that can be mounted on a wall or on the back of an LCD. The power usage of this computer ranges from 15 to 21 watts depending on its load, so I stand to save $30-$40 a year in electricity when compared with a more typical computer that consumes 50 to 80 watts (I leave my computer running 24/7).
There are prebuilt computers that are similar in price, such as the Zotac MAG, but they use proprietary parts. That means if something breaks outside of warranty, you may find it difficult or impossible to find replacement parts. With a standard mini-itx system like this one, parts availability should not be a problem for several years.
Speaking of “green”: did you know that you can save money on ink simply by using a different font when composing documents for printing? According to this article on Yahoo! News, a font with thin lines such as Century Gothic can use up to 30% less ink than popular fonts such as Arial. If you’re using Linux instead of Windows, you may not find either one of these in the list of fonts available to you. However, the concept still applies; just look for a font with thin lines.