I have a new Asterisk server at home. It is a Contec IPC-BX/M600(PCW). This is a wall-mount PC. It is about a foot square and about two inches thick. There is no power brick. The power cord plugs directly into the box. It has a 400MHz processor which should be fast enough for home or small-office use. It has only 64MBytes of RAM, so you need some swap, particularly when compiling Asterisk.
I bought this marvel new on E-Bay for about $55 including shipping. I added a laptop hard disk for storage.
Power consumption is reasonable, about 15 watts when idle.