I've been getting by with using Workgroups throughout the years. I'm wondering if there is a free lightweight solution that doesn't involve setting up a Windows Server domain controller?
You don't have to set up a Windows Server domain controller specifically, but you will need some sort of server that is always on to manage authentication and such. Apache Directory is an open source solution which is similar. It offers LDAP support and Kerberos authentication as well as a number of tools for administration via Apache Directory Studio. It is not a replacement for a Windows DC but it offers some similar functionality. It depends on what features you need from a domain controller.
I agree w/ mfini, the desired functionality and why you feel AD isn't a good option will help with our suggestions.
Active Directory is lightweight and free w/ the OS; creating the same functionality w/ a different product is likely to be more and not less complicated/lightweight. Samba is a high functioning alternative, but not necessarily an easy one. Novell has it's own directory, also not necessarily more or less lightweight than AD, also not free.
MyLogon is lightweight and may be an option depending on the functionality you need, it's not free though. I haven't used it but I have seen it mentioned a few times. http://en.wikipedia.org/wiki/Mylogon
MS Foundation Server is a cheap option for <=15 users, must be purchased w/ hardware.