DistrictBuilder is web-based, open source software for collaborative redistricting.
From the website:
DistrictBuilder was developed in collaboration with leading redistricting experts at the Public Mapping Project. Whether you work for a state or local government agency, an advocacy group or you are a legislator, DistrictBuilder has been designed to provide you with affordable, easy-to-use, customizable redistricting tools that make the redistricting process more open and collaborative across agencies and with the public.
DistrictBuilder is part of the Public Mapping Project.