Most people seem to find the idea of a computer being "green" a bit of a paradox. Technology just seems to be one of those necessary evils, something that just needs to consume whatever amount of energy it needs to, and be made of whatever materials it needs to be made of in order to work. And you want it to work, right?
Companies like Dell have recently gotten on the "green" wagon by marketing their new Optiplex models as being more energy efficient than previous desktops in that line have been.
Also, they advertise that they use less heavy metals, like mercury and lead, in the manufacturing process, and when you order one, it comes preconfigured for maximum energy savings.
There's even an option to plant a tree.
If you're thinking about buying a new computer, but aren't sure what your options are, go to climatesaverscomputing.org, where you can find a list by manufacturer of the latest in energy efficiency and low-impact desktops and laptops.
One company which isn't mentioned on Climate Savers, is enano. I've gotten to play with one first hand, and I have to say that they're designed for power computing, and extremely sexy.