Mystic Theurge

The Mystic Theurge is a Prestige Class from Dungeons & Dragons 3rd edition and the Pathfinder corebook. It can basically be summarized as "I want ALL the motherfucking magic!"

Seriously, that's all there is to it. A Mystic Theurge is a PRC that improves your capabilities as both a Divine Spellcaster and as an Arcane Spellcaster. Great if you really can't choose between being a Cleric and a Wizard, or an Oracle and a Sorcerer, or whatever.

To get into this Prestige Class, you need to be able to cast at least 2nd level Arcane spells AND 2nd level Divine spells. You also need either 6 (for D&D) or 3 (for Pathfinder) ranks in BOTH Knowledge (Arcana) and Knowledge (Religion). In return, you get a ten level PrC where each level gives you spellcasting improvement in both your Arcane spellcasting class and your Divine spellcasting class.

The Pathfinder version is a little meatier, in that it gains access to the Combined Spells class feature (which basically lets you give up one class's spellslot to prepare a spell from the other class of one level lower) and the Spell Synthesis class feature (which lets you cast an Arcane spell and a Divine spell simultaneously once per day).