Intel's NUC is built around a standard motherboard form factor that allows others to create their own mini PC enclosures. Traditional case makers haven't gotten in on the action, but smaller boutique builders are making some really cool stuff, including passively-cooled chassis that do away with the NUC's laptop-style blower. My favorite so far has to be the Cirrus7 Nimbini:
Developed in Germany, the Nimbini trades active cooling for a series of "modular aluminum layers" that dissipate heat from the ultrabook-class Broadwell processor buried within. Turning the case into a heatsink calls for larger proportions than typical NUCs, but the 5.9" x 5.9" x 3.4" dimension are still relatively compact considering what's inside.
In between the chunky metal fins sits an Intel motherboard with Core i3, i5, and i7 processor options. The system can accept M.2 storage alongside a 2.5-incher, making it easy to combine solid-state and mechanical drives. Connectivity includes four USB 3.0 ports, two Mini DisplayPort connectors, and one Gigabit Ethernet. Sadly, there's no HDMI out for big-screen TVs.
When it ships next month, the Nimbini will be available as both a barebones config and a full system. The prices haven't been announced, but given the sticker on Cirrus7's passively-cooled Haswell rig, expect to pay a steep premium over more conventional mini PCs. Thanks to FanlessTech for the tip.