Update: Version 1.0.4 is here! It now supports Project IceStorm. Note that the updated in 1.0.0 is broken for Windows so you need to manually download and install 1.0.1 or above. You can download it below.
You can download the Windows drivers here.
If you are running Linux, make sure to move the 99-alchitry.rules file from the driver folder into /etc/udev/rules.d/. This will prevent the board from showing up "unknown" when listing them off.
To build projects for the Au you need to install Vivado (WebPACK, aka free).
To build projects for the Cu you need to install iCEcube2 (bottom of the page).
If you are building for the Cu, you will need to point the IDE to your license file for iCEcube2. You can get a license here.
As promised, the source is here.
Version 1.0.2 introduced Alchitry Constraints! These allow you to specify the pin locations based on the Alchitry header numbers instead of the FPGA pin numbers.
Check out the new built in demo projects for the IO Element for a demo of the constraints.