The Mages Guild is perhaps the handiest guild in the whole game. Some of their services (Identifying magic items and selling pre-made spells) are available to non-members. Every player who can join the Mages Guild should do so, if for no other reason than gaining access to the spell-maker.
Benefits of Membership:
Mages Guild quests tend to fall into a few general varieties: stand guard for 3 hours (you get to fight thieves and assassins), fetch research papers, kill imps, kill runaway lab experiments, fetch spell components, kill rogue wizards. The last ones involve dungeon crawling, but the rest are pretty straight-forward.