"Guilds and Adventurers" by Various
Published and distributed by Mystic Eye Games
ISBN = 0-9708265-6-7 128 pages
This older book was supposed to be part of Mystic Eye's "The Hunt: Rise of Evil" campaign. In its 44 small chapters, are details for 30 different guilds. There's also a section on other groups.
The Prestige Class presented is better off used as an NPC. But where this book really shines is in the descriptions of the 30 guilds presented. Everything from an Adventurer's Guild to a Weaponcrafters Guild has at least a couple good pages of design notes. Although the art is black and white, it is good, and complements the text nicely. About 70 pages of the 128 pages are devoted to guilds. The remaining pages give details on Associations, Societies, and Specialty Groups.
Every guild presented also has an icon associated with it which could be useful when in a strange city with a strange language. The crests are universal to the guilds. There are also tables and charts scattered throughout the book in places where they relate to a specific guild. New feats, new skills, new items, even a new monster or two, and new spells are also included.
I could not find anything in this book that could not be used in almost any campaign, although the book is designed for D20. It's too bad Mystic Eye Studios is no more. So far, I've come to really enjoy almost everything they produced. GM's - even if you're running, say, Pathfinder, you will probably find this book to be incredibly useful. Highly recommended even just for campaign city flavor.