The Master and Commander fandom consists of fans of the movie Master and Commander: The Far Side of the World, and the Aubrey/Maturin books by Patrick O'Brian, on which the movie was based. Most communities were created at movie/DVD release time, but discuss both movie and book versions of the canon.
Some non-M&C-specific Age Of Sail communities and information
despatches - Age Of Sail fandom newsletter community, with a regular roundup of Age Of Sail-related posts anything_aos - a general Age of Sail fan community aos_challenge - Age of Sail fanfic and art challenge community kiss_me_hardy - historical discussion for Age of Sail fandoms aos_resources - links to Age of Sail information/sites tallships_daily - pictures of tall ships
Selected non-LJ sites
Master and Commander Slash Mailing List - a Yahoo Groups mailing list for Master and Commander slash The Gunroom - high volume, long-standing mailing list, with mostly book-focused discussion (they say their two topics of discussion are "Patrick O'Brian and everything else"). Patrick O'Brian Web Resources - affiliated with the Gunroom, this is a long list of POB-related web sites Historical Resources for Fanfic Writers - a good resource for writers in any of the historical fandoms A Patrick O'Brian Literary Guide - "A brief introduction, for fans of Peter Weir's 2003 movie, to the characters and plots in Patrick O'Brian's original books." Maturin's Medicine - a guide to the medical terminology and cures that Stephen used on his patients
Let me know if I've missed or misstated anything. Thanks for reading!