I guess it depends on what you need from the server...
For a simple file server, samba
is an easy, cross-platform way to go (so long as you follow the instructions and do a bit of reading). If you're 100% unix systems, you could use NFS instead, which I'm told is better (no experience with it myself, it's on the list for another time).
From the website, it looks like Mirra's main claim to fame though is its backup facility - they have their special client software that mirrors everything you save on the server. This would be hard to get right with free tools (I think) You could write your own client software that monitors disk accesses and only copies those files, or you could just do what I do, and put everything on your file server (map a drive in windows and make it your "my documents" folder).
If you want to synchronise files between multiple machines, then the best "poor man with some effort" option might be cvs or svn - at least for text-based files, as you can make changes on different machines without being up-to-date and merge the results (and occasionally fix conflicts). I really like having my home directory in cvs, even for binary files, but I don't normally need to make major changes from multiple locations.
If you describe exactly what you want and how you want it to work, there might be some more specific ideas coming out.