you can buy kits that will let you run an external power brick to run a standard pc, long as your power requirements are low enough.
http://www.logicsupply.com/product_info ... cts_id/255
http://www.logicsupply.com/product_info ... cts_id/254
Also, alot of Mini-Itx cases use external power bricks to power them.
http://www.logicsupply.com/product_info ... cts_id/695
http://www.logicsupply.com/product_info ... cts_id/107
There are threads in the power supply forum about using a 220w 12v external brick for higher powered systems. The same one used in the SD11G5 shuttle cube. But modifications are required as so far i havnt seen it for sale in retail channels yet.
220w is the highest wattage passive external brick i know of, and to my knowledge only 1 company makes it.
180w passive bricks are alot easier to find, but still need modifications as i dont know of any retail versions being sold.
150w and below are redily available from alot of retail channels. Some have fans though, so be carefull.
The standard input voltage for most DC/DC adapter boards to make these external bricks work with a standard computer are usually 12v.