Best FOSS AD alternative?

danboid writes: "I'm an IT technician at a large school near Manchester, England. We currently have two separate networks (one for pupils, one for staff) each with their own Windows 2003 AD server handling authentication and storing users files. We're planning on restructuring the network soon and we'd like to be able to replace the two ageing AD servers with a single, more powerful Linux server running an open source OpenLDAP implementation. Fedora Directory Server, OpenDS and Apache Directory Server seem to be the main contenders for this purpose but I've been unable to find a comparison of the three. I'd like to hear which solution slashdot readers recommend, ease of implementation/ maintainance, any stories of any similar (un)successful migrations and any other tips for an organisation wanting to drop AD for a FOSS equivalent."