Well if you want single sign on with linux or a single repository for users you can use Radius I believe, not sure how Windows would use it though. The Domain way could work by creating a windows domain and windows Radius server for the Linux servers/clients to read. Another option is to use LDAP or OpenLDAP, again not sure how windows would use open ldap as a login point. We use Novell OES which integrates LDAP with eDirectory(user repository) and has a client for Windows and that does work pretty well. So there are several ways, if you are looking at all free stuff I would look at open ldap and integrating your Linux servers with your Windows servers via Samba. I have never personally done this so I am not 100% positive how it would work but from reading through stuff it seems that would be a way to go.
Hope that helps some