The ASRock M8 has just arrived in the office and it's one of the best-looking mini-ITX boxes I've seen in a long time. It's a high-end barebone PC, which means you'll need to provide your own processor, graphics card, memory, cooler and storage, but it uses a PCIe riser board so you can lie your dual-slot graphics card in line with the motherboard.
In spite of the riser, you couldn't really call the M8 barebones small , and it's costly, too. Prices start at £413, which includes the chassis, fans, ASRock Z87-M8 mini-ITX motherboard, a slimline optical disk drive and a 450W SFX power supply.
It may not be in the realms of SilverStone's Raven RVZ01 , in terms of getting as small as possible while still allowing for dual-slot GPUs, but it's still a pretty small form factor machine when you compare it with a full tower PC. The M8 has enough space to house some high-power components, so you're unlikely to notice any lack of performance compared to a full tower PC either.
As well as a beautifully rugged Steam Machine, you could build an excellent LAN party machine into it too. It may not be particularly light, but that chassis - co-designed with BMW DesignWorksUSA - is seriously solid and comes with a selection of carry handles at each corner.