If you're doing a custom case and don't limit your imagination, you can use one of the many netbook or laptop motherboards. Downside is they're limited to 1-2 m

If you're doing a custom case and don't limit your imagination, you can use one of the many netbook or laptop motherboards. Downside is they're limited to 1-2 mPCIe slots, on the other hand they also have 1-2 SATA ports so you can easily connect decent storage. Plus USB and video ports. Some boards have stupid whitelists for the mPCIe slots, so a bit of research is desirable. Or just steer clear of HP/Dell/Lenovo. Low power netbook chips can be passively cooled, and power bricks and connectors aren't as much of a problem as you'd think. They can be had for as low as $10 on eBay since barely anyone buys them. Kind of a shame.