I've long belabored the importance of 'access' in game balance and since I did so with the last post, I thought it'd be a good opportunity to share where the idea came from. Throughout Third Edition I slowly built a magic system that achieved my goals. Basically, I wanted to let a wider range of magic using characters exist and be fully balanced without requiring an ever ballooning list of classes, spell lists, and spells.
This system still has its issue, but with about 100 spells it it lets any class become a casting class in any multi-class combination AND decide to become a caster at any point in their career. An 8th level fighter picking up a level of mage made a choice--he didn't sacrifice a bunch of power to become over powered later. Over time, I started trying to distill why this worked down into simpler tradeoffs and that evolved into the current proficiency system.
Rather than re-write everything up, I figured I'd just link to the digital graveyard where the magic system resides. The link goes to the flavor text portion of the tab, the meat of the content is accessible from the sidebar below MAGIC in the sub-tabs of 'Forms,' 'Mage Class,' and 'Magic Mechanics.'