The mac g3 cube was fanless.
There are tons of industrial/hahahahahaded PCs that lack fans, but most are in the sub-gigahertz range, P3 pr celeron technology, and aren't as expandable as you'd expect with typical consumer or desktop PCs...plus they cost a lot more. Usually they use solid state disks, too, but if they're PC/104 compliant you can add an IDE disk controller.
Here's a 400 MHz fanless with decent expandability for $1200:
http://www.stealthcomputer.com/littlepc_fanless.htm
The whole case looks, and acts, as a heat sink.