I've become enamored with quiet, power-sipping, small form factor computers for the home. I like them because they are unobtrusive visually and audibly, yet can be always on without major guilt over the power consumption.
If quiet is the only goal, then the End PC Noise folks seem to have a lock on the quiet PC accessories and systems. Their systems are not typically fanless nor particularly low power. They do have a lot of products to quiet your desktop PC. But my major home applications don't require tons of computing horsepower, so there's no need for every computer in my home to be heavy iron. Here's where the small and efficient computers come in.
Of course there's small laptops and netbook form factors, which are nice and stowable, but there are other candidate form factors that are more efficient, smaller, and sometimes more powerful.
fit-PC2 Available this month. Fanless. Less than $400 fully configured. Can stream HD video with 10-40% CPU load. Atom based.
ZaReason has the 'Breeze 3660' for $350 which consumes 30 W.
I've read that Shuttle will soon make an external power supply version of the Shuttle KPC, which would eliminate the pesky fan noise.
Local suppliers PC Liquidators have inexpensive micro and mini -ATX cases and the like, in case you want to make your own.
Any number of small linux distributions are available on lightweight computers.
DamnSmallLinux is one example that seems close to a best of breed. There's also Cappuccino and a personal favorite (for no apparent reason other than the product seems like good value) Z-Tech Services.