We are currently using ahsay to run backups on end user systems that backup to our specified directories within the server. My question would be what is a good open source backup solution that is similar in the aspect of having a service run on an end users machines that communicates with the server?
You don't specify the operating system, but if you are using Linux & the GNOME Desktop I recommend Déjà Dup.
More details re: your configuration would be extremely helpful, but Amanda Community Edition supports the client/server model I'm assuming you're looking for and has agents for Linux, Windows, and with some work, Mac OS X.
Amanda also has a BackupPC solution with deduplication that looks to be a bit more lightweight for desktop backups, but I haven't tested it so I can't comment on it.