The server software supports various functions including a mail server and even remote Xcode compilations. However sometimes it's useful to remotely access the Mac Mini using Apple's remote desktop so getting a virtual screen on to the unit itself. Unfortunately if it's in headless mode, the on-board GPU is not enabled and all graphics is handled by the main CPU, which can make the system seems extremely slow as the CPU is spending it's time rendering the screen, animations and doing screen refreshes etc.
Now there is a solution to this and Newertechnology have produced an HDMI Headless Video Accelerator (t's about the same size as a small Bluetooth or WiFi adapter) that is plugged into the HDMI port and then the Mac Mini then thinks a screen is attached and thus the GPU is enabled meaning all screen handling is done by the GPU rather than the host CPU and everything runs smoothly again.
The adapter supports a maximum resolution of 1080p (and up to 3840 x 2160 on a late 2014 model). Other models supported are Mid 2010 through to the latest. OS X 10.6.8 is the earliest version of the operating supported (no drivers are required).
It can be found on-line for around £21.99. A really useful little edition if using a Mac Mini in headless mode and accessing it remotely (it's also true for using it for remote animation and anything that uses the GPU).