I've Got Your Sword-Wielding Wizards Right Here!

Idea for a magic system:

Ingredients:

1. Enough non-magic-using classes that you think the campaign will be interesting. How many counts as interesting obviously depends on you and your players.
2. A spell list.

Instructions:

Only non-magic-using classes are available. When stocking treasure, increase the chances of magic scrolls being found by at least 2, maybe more, depending on just how much magic you want the PCs to have access to.

When characters find magic scrolls, they can choose to read them. If they read them, the magic spells leap from the scrolls into their heads (erasing the scroll). They now know the spell and can cast it, as with standard Vancian magic. Casting a spell memorized in this way erases the spell from memory, as with standard Vancian magic. There is a 1/6 chance that such spell scrolls are labeled so that characters can determine what the spell is before reading and memorizing the scroll. 

The number of spells that can be held in memory at one time is limited by class level. The simplest way to do this is to have the number of spells memorized be equal to or less than the class level.

Whenever the character levels up while having one or more spells memorized there is a chance (a Wisdom check, or # of currently memorized spells/20, say) that one of the memorized spells (select randomly) will "stick" and the character will be able to cast the spell indefinitely. The simplest way is to allow each "permanently memorized" spell to be cast once a day.

Ways to make this more complicated:

• Labeled scrolls may be mis-labeled
• Stipulate that spells of spell level higher than class level may not be memorized
• Stipulate that total spell LEVELS memorized must be equal to or less than class level
• Bolt on some sort of damage or sanity mechanic, or some other consequence, if memorizing more spells or spell levels than is allowed is attempted; if spell levels rather than spells are being counted, this means that memorizing an unknown spell is always a risk
• Alternately, stipulate that the only way to read a spell (and figure out what it is) without memorizing it is to read it while already having memorized the maximum number of spells currently allowed to be memorized
• Introduce costs to casting a spell, such as hit points or attribute points, which replenish at some rate
• Stipulate that how often a permanently acquired spell can be used depends on class level and spell level ratio. For example:
• A spell that is 4 or more spell levels below the character's class level is an at-will spell
• A spell that is 2 or 3 spell levels below the character's class level can be cast 3 times a day
• A spell that is within 1 spell level of the character's class level can be cast once a day
• A spell that is 2 or 3 spell levels above the character's class level can be cast 3 times a week
• A spell that is 4 or more spell levels above the character's class level can be cast once a week
• If using a system like this to determine how often a permanent spell can be cast, choose between having the frequency with which a particular acquired spell can be cast either always stay the same as it was when first acquired or increase as the character levels up
• Bring in a second spell list. The first list is for the spells that can be found. The second list is rolled on to determine permanently acquired spells. Arcane spells could be found, but Divine spells acquired, both Arcane and Divine spells could be found, but psionic powers are acquired, etc.