The Freeport Companion, published by Expeditious Retreat Press for Dungeons & Dragons 4E.
Credits: I actually only sort of worked on this book, in as much as they used/adapted some of my material from the Advanced Player’s Guide. But I’m a huge fan of the Freeport setting, and my name’s in it, so I’m happy to add it to my list. 😉
Notes: This book includes the 4E-compatible mechanics and material to support Green Ronin’s system-free Pirate’s Guide to Freeport.