There are two flavors of Universal Type Server - Professional, which is scalable to any size workgroup, and Lite, for workgroups of up to 10 users. The server and client applications are cross-platform, running on Windows as well as Mac.
Migration paths are available for users of Font Reserve Server or Suitcase Server. The Lite package is available for $1395 directly from Extensis, but you'll want to contact a reseller for the Professional version. If you want to kick the tires before you buy, you can download 30-day free trials for both Pro and Lite.