Clustering On *BSD?

Lonesome asks: "I'm looking at clustering several Free/Open BSD or Solaris servers in a production environment for high availability. i.e. the machines are linked together, for all purposes look like one machine and the clustering software takes care of keeping the data on the machines in sync (they'll be handling file [including NFS] and mail services) so that if one of them falls over the others will still provide the services, users won't see much difference and minimal data will be lost. All I have been able to find in the way of clustering tools is either in early development or vastly expensive. Does anyone know of a good, preferably BSD/GPL license, solution?"