A CLI tool for indexing and searching very large Maildir/ structures (like list archives). Tries to be very fast (2 million messages take less than 3 minutes on a 12 core machines to fully index). It's in the early stages of development right now, so don't expect much.
Mairix is software for indexing and searching your e-mail queues. Can handle Maildir/, MH, and mailbox mail queues. Runs incrementally on every new e-mail. Search results go into virtual mail folders in your mail client. Extremely fast.