1) export DISPLAY=:0
for *nix or setx DISPLAY ":0"
for Win.
Windows Server 2008 R2 + AMD R9 290 - VNC works. Windows Server 2012 R2 Update1 + R9 290 Ultra VNC mirror driver is required also. On OS X 10.6.8 I could not manage export display to work.
VNC is working, but resolution is limited to 800x600 on 2k8 and 640x480 on 2k12. Same resolution with TV.
Also when connecting to locked 2012 + R9 290 VNC shows black or purple screen, however after sending "ctrl+alt+del" and entering password(blind) VNC shows desktop. That's probably because 640x480 is too low for Metro.
2) For Windows 7 with some GPUs(deffo works for AMD 6950/6970/7950/7970/280x), Windows 2008 R2 + Intel HD 4000 and for Windows 8.1 + Intel HD Graphics 4000 - force display works for me - https://lbr.id.lv/coin/ForceDisplay.html Resolution will be limited also, but not to 800x600. 2008 R2 / 8.1 + Intel 4000 - 1280x720.
Microsoft driver for Intel 4000 didn't allow me to "force", but Intel's driver did.
3) EDID dummy - http://www.ebay.co.uk/itm/DVI-D-Digital-Dummy-Plug-Bitcoin-Litecoin-mining-1920x1200-R9-290-headless-/201210868272?pt=LH_DefaultDomain_0&hash=item2ed91a3230 or http://www.ebay.co.uk/itm/DVI-DVI-D-dummy-plug-DDC-EDID-emulator-fake-monitor-headless-R9-290-290X-/161282393371?ssPageName=ADME:L:OC:CA:3160
This one I use on Mac Mini with OS X 10.6.8
4) Some ppl claim HDMI to VGA adapter also works, however I did't try it - http://www.ebay.com/itm/HDMI-Male-to-VGA-RGB-Female-HDMI-to-VGA-Video-Converter-adapter-1080P-for-PC-FO-/291326539661?pt=LH_DefaultDomain_2&hash=item43d46a378d
Also any active hdmi-to-something adapter will work, like KVM switch etc. And if you connect with MS RDP you can use whatever resolution you like, but OpenCL or any other GPU specific stuff won't work.