I've no experience of them and have only read the odd bit myself.
Essentially you use one well specced PC (or Laptop) as a central server.
You then connect clients to the network to give additional 'terminals'. These clients take various forms but essentially the server runs all the software and does most of the work for all the 'network'. This means the clients can be much lower spec, no need for a full OS, no hard disc, very little memory, they really just allow you to connect an additional screen,keyboard and mouse to the server (laptop) so that more than one person can use its processing power.
With Ultra Thin Clients the client can use as little as 4w !! (plus your monitor of choice).
So three PCs become one PC plus two 4w clients!
Five PCs become one PC plus four 4w clients.
There is a catch of course. They can only be used for fairly basic tasks, no latest games or video editing, so you need to check your intended usage against their capabilities.
Sun seem to be the major players from looking around on the net at least.
http://www.sun.com/sunray/sunray2/features.xmlhttp://www.sun.com/software/index.jsp?cat=Desktop&subcat=Sun%20Ray%20Clients&tab=3Secondhand available on Ebay ....
£49.99 each ( includes monitor! around 30w - 40w all in I think)
http://cgi.ebay.co.uk/Sun-Ray-150-Thin-Client-Keyboard-Mouse-SunRay_W0QQitemZ110349171736QQcmdZViewItemQQptZUK_Computing_Networking_SM?hash=item110349171736&_trksid=p3286.c0.m14&_trkparms=72%3A1690|66%3A2|65%3A12|39%3A1|240%3A1318£124.99 for a sunray 2
http://cgi.ebay.co.uk/Sun-Ray-2-Thin-Client-Keyboard-Mouse-SunRay_W0QQitemZ110349255340QQcmdZViewItemQQptZUK_Computing_Networking_SM?hash=item110349255340&_trksid=p3286.c0.m14&_trkparms=72%3A1690|66%3A2|65%3A12|39%3A1|240%3A1318Job lot of 25 starting at £50
http://cgi.ebay.co.uk/SunRay-1-380-0299-07-Ultra-Thin-Client-x25-joblot_W0QQitemZ160322436537QQcmdZViewItemQQptZUK_Computing_Networking_SM?hash=item160322436537&_trksid=p3286.c0.m14&_trkparms=72%3A1690|66%3A2|65%3A12|39%3A1|240%3A1318Lots of further research needed of course but interesting to look at in terms of low power / low heat output.