Ones that are resources for the PCs:
Mercenaries' Guild: A place to look for jobs and a training resource for all fighting arts. Maintains a hostel in each large city where members can get food and lodging. Of course, members may have obligations to go along with the benefits.
Mages' Guild: Training source for all things magical, a source for new spells, and possibly a source for rarer spell components. Also responsible for maintaining certain enhancements to larger cities such as water purification systems. Members will have obligations as well as benefits.
Churches and the Druids: To clerical characters what the Mages' Guild is to wizards. Again, obligations to go with the benefits.
Resources for DM and sometimes PCs:
Thieves' Guilds: Training, equipment, plot hooks, recurring enemies, potential allies.
Assassins' Guilds: as above but in my campaign more about plot hooks and potential/recurring enemies (all good aligned PCs).
Intelligence Services: our current rogue is actually an operative of Elven Intelligence. Training and equipment are provided by them to her similar to how Thieves' Guilds and Mercenaries' Guilds work for other characters. She also is expected to report as often as possible on her findings/doings and may occasionally receive assignments (plot hooks and/or side quests).
My personal favorite, original to Melkar:
The Kudaji: Among gnolls, when a child is born with all-black fur it is seen as the Mark of Yeegnoghu (sp?). These children are trained in the god's service differently than his normal clerics and are the gnolls' Special Forces and assassins. All are trained as either fighter/clerics, cleric/thieves, or the occasional triple class covering all three.
A Kudaji of just a level or two ahead of one of my PC groups made a great/maddening recurring enemy for years of play. One of the players from that group will start cursing at any mention of the name Wernak to this day
