Plain text. Put one tag per line in each text file. e.g "topic:", "subject:",etc. Use grep and awk to slice and dice as and how you want. Incredibly flexible,

Plain text. Put one tag per line in each text file. e.g "topic:", "subject:",etc. Use grep and awk to slice and dice as and how you want. Incredibly flexible, no lock in, no dependencies, easily replacable parts. Check into git regularly.