WikiPOBia. The Patrick O'Brian wiki.
Aubrey/Maturin series text search. Enables you to search the entire canon.
Patrick O'Brian Web Resources (Wayback Machine). Gibbons Burke's PO'B links page was wonderfully comprehensive. Sadly, it is no longer being maintained, so some of the links no longer work, but there is a lot of good stuff in there.
The Dear Surprise. Resources to enhance the reading of the canon.
Patrick O'Brian at W. W. Norton. Norton's PO'B publications.
The Patrick O'Brian Compendium. PO'B sites and information assembled by Tony Townsend.
The Patrick O'Brian Squidoo Page. A collection of Patrick O'Brian links.
A List of POB Companion Books (Wayback Machine) on the Amazon.com website, compiled by Bruce Trinque.
The Nautical Fiction List by John Kohnen.