Apple's Xserve computers were a rack-mountable line of Macintosh systems designed for enterprise use; however, in 2011 Apple abandoned the project, leaving only its Mac Pro and Mac Mini with available server options. Though anyone can download the OS X Server tools from the Mac App Store and convert any current Mac into a server system, some may wish to get a rack-mounted setup going again.
Fortunately, even though the demise of the Xserve means there aren't officially supported rack-based options from Apple, there are some ways you can get your Macs back into the rack. Of course, one option is to simply place a couple of Mac Pro or Mac Mini systems on a standard rack shelf or drawer (such as the MK1 Mac Min Rack Adapter or H-Squared Mini Rack). However, if you want a dedicated rack-mounted unit, there's Sonnet's xMac Mini Server.
This device is essentially a rack tray for Apple's Mac Mini, but in addition to a built-in power supply and pass-through connections for the Mac's built-in inputs, it offers PCI Express expansion through a Thunderbolt connection to the Mac Mini. You can place a Mac Mini (purchased separately) in the xMac Mini Server, attach the power, Thunderbolt, and extension cables for the Mac's built-in Ethernet, HDMI, audio, and USB ports, and then slide the whole unit into a standard rack.
While Sonnet's xMac Mini Server has been available for a while, it has recently been updated to support the same full-length, full-height x16 PCIe card along with another half-length x8 PCIe card, but in addition include BNC connector support and brackets for mounting a daughter card. The updated system is also reduced in length, and has three USB 3.0 ports, one on the front of the chassis, and a Thunderbolt pass-through port for additional Thunderbolt-based expansion devices.
With this setup, using a number of compatible PCIe cards you can use your Mac Mini for a number of different server and controller configurations, whether to create a simple storage manager or a media-processing center.
The xMac Mini Server is a bit pricey at $1,295 per unit so it may not be suitable for everyone, but it does fill in that rack-mount gap in Apple's lineup, and gives your Mac Mini expansion capability with PCIe options.