Here's one from my bookmarks used in the medical industry. I know these are not exactly practical but it CAN be done, I actually built a PC that was silent by using a horizontal case and putting heatsinks on absolutely every part, then if you want you can place a couple 120mm fans that are set EXTREMELY low and the thing won't overheat. It's definitely possible and is done all the time in recording studios as the sound from a computer is terrible in that environment!
Fanless PCs - The transtec AG - Customized Hardware & Complex IT Solutions