My group play-tested the original version with 2e /Diablo and while it was fine for Diablo it was a bit overpowered for standard D&D. We found there was too much magic missile spamming and in games that were less undead heavy, there would almost certainly be too many sleep spells for game balance.
However this version is better balanced and should be OK for most games. And in case you wondered, I actually prefer the default Vancian system a. Its more flavorful and more D&D in my opinion.
Now what this system does is grant spell casters a bit more flexibility, a bit more power and eliminates the "15 minute adventuring day" . Its quite possible to adventure all day and manage resources with a lot more precision.
It comes at a cost in blandness and more importantly in game balance. This will make spell casters stronger and as such, even in old school games some kind of gimme for non spell casters might be a good idea.
So without further ado, Spell Points ala 5 Stone.
OSR and Pathfinder version.
Add all your spells per day together.That is your spell point pool.
You know whatever spells your DM allows and may cast them as per normal however all spells that directly inflict harm effects on people other than the caster cost Level +1. Other spells cost an amount equal to their level. when the pool reaches zero, you can no longer cast spells. You regain the pool with 10 hours uninterrupted rest.
Findel a 5th level Int 14 Elf Fighter/Magic User will have 10 spell points (3 1st level spell, 2 bonus spells for INT 1 L2 spell and 1 L1 spell ) He knows sleep, mount , shield, charm person, light, magic missile, web and fireball
He can cast sleep for 2, mount for 1 , shield for 1, charm person for 2 light for 1, magic missile for 2 web for 2 and fireball for 4
This certainly gives him bonus flexibility but still keeps him within reasonable limits. Now note it can prevent low level, low stat magic users or clerics from actually casting damaging spells but thats a feature not a bug.
For a lower magic feel, you can double the cost of person effecting spells instead (sleep 2, web 2, Fireball 6 )
#1 Pathfinder . You will need to exclude L0 spells in Pathfinder as these have unlimited use.
#2 Older editions. I recommend allowing high INT magic users bonus spells as per clerics in these versions as 1st level magic users will not eb able to cast direct damage spells or sleep without the extra points.
#3 3.5 version. The 3x version works identically however as cantrips are daily spells, the spell pool is calculated at Level +1 for all spells (with L0 spells costing 1 point) and spells cast from the pool at L+1 and L+2 for harmful magic ...
#4 Optional rule. If you want faster or slower recovery, you can change the recovery rate. The most popular change is 10% per hour. This is a bit of a power up so be warned!