Just for variety, here is the system I have been working on:
Spell casting
Spell Memorization Slots
A caster can have a total of 8 spells 'on tap' at one time (called spell memorization slots); these are the only spells that caster can cast, though they may know many more spells. Removing a spell from one of these 8 spell slots is a free action, but adding a spell takes 20 minutes per spell level of meditation. In addition to these 8 slots, clerics and druids can cast any spell that they can spontaneously cast. Clerics also get a 9th slot which can only be filled by one of their domain spells (domain spells can also go into any of the 8 main spell slots). Specialist wizards get a 9th spell slot that can only be filled with a spell from their specialty school.
Spell casters get mana points each level. These can be used to cast any spells - there are no spell slots. There is no difference between a sorcerer and a wizard (ie: sorcerers are deleted). 10% of a caster's mana points are regained after a full hour of rest, sleep or meditation. Multiclassed characters have one pool of mana points, which can be used to cast any spell they know.
Mana Points per Level
The first column is for wizards, druids and clerics; the second is for bards, and the third is for paladins and rangers. (So a Wizard 5/Cleric 1 has 12 mana points, 11 from wizard levels and 1 from cleric. If the 5th level wizard instead become a Wizard 5/Incantatrix 1, they would have 16 mana points).
cleric/wizard/druid
Level mana gained (total mana)
1 +1 (1)
2 +1 (2)
3 +2 (4)
4 +3 (7)
5 +4 (11)
6 +5 (16)
7 +6 (22)
8 +8 (30)
9 +9 (39)
10 +11 (50)
11 +13 (63)
12 +16 (79)
13 +19 (98)
14 +23 (121)
15 +27 (148)
16 +32 (180)
17 +37 (217)
18 +43 (260)
19 +49 (209)
20 +57 (266)
bards
Level
1 0
2 +1
3 +1 (2)
4 +2 (4)
5 +3 (7)
6 +3 (10)
7 +4 (14)
8 +4 (18)
9 +5 (23)
10 +6 (29)
11 +7 (36)
12 +8 (44)
13 +9 (53)
14 +10 (63)
15 +11 (74)
16 +12 (86)
17 +13 (99)
18 +14 (113)
19 +15 (128)
20 +16 (144)
paladin/ranger
Level
1 ---
2 ---
3 ---
4 0
5 0
6 +1
7 +1 (2)
8 +1 (3)
9 +1 (4)
10 +2 (6)
11 +2 (8)
12 +2 (10)
13 +2 (12)
14 +3 (15)
15 +3 (18)
16 +3 (21)
17 +3 (24)
18 +4 (28)
19 +4 (32)
20 +4 (36)
Bonus Mana for High Ability Scores
For characters that multiclass or take on a prestige class with spell casting, bonus mana is received only once.
Ability Bonus
12-13 +1
14-15 +2
16-17 +4
18-19 +7
20-21 +11
22-23 +16
24-25 +22
26-27 +29
28-29 +37
30-31 +46
32-33 +56
34-35 +67
Mana Cost by Spell Level
Spell level (cost): 0 (0); 1 (1); 2 (2); 3 (4); 4 (7); 5 (11); 6 (16); 7 (22); 8 (29); 9 (37)
(so to cast a 5th level spell costs 11 mana points)
Spell Availability
All casters must learn spells before they can cast them, however they do not need to keep spell books. Casters can learn 3 spells at 1st level (that is, the 1st level that they gain spellcasting), and 3 spells for every level gained (levels in a spell casting class). In addition, any character that can cast 0 level spells can learn up to 6 such spells for free (additional 0 level spells can always be taken from the 3 spells per level). The level at which they gain spell levels is the same as in the Player's Handbook (ie: wizards, druids and clerics gain 0 and 1st level spells at 1st level, 2nd level spells at 3rd level, 3rd level spells at 5th level, etc.). Casters cannot learn a spell that they are unable to cast. Any spell that can be cast sponaneously is automatically learned, in addition to the 3 spells per level. Specialist wizards can also learn one spell per level from their specialty school.
Spells that can be learned are the maximum number but, except for spontaneously cast spells, they are not learned automatically. The caster must learn spells from scrolls, or be taught the spell by someone who knows it. Spell slots can be saved, but they cannot be filled with spells higher than those that could have been learned originally (ie: a 2nd level wizard learns 2 spells only before leveling to 3rd level; they can now learn 4 spells, but the 1 saved slot has to be filled with a 0 or 1st level spell).
A caster can unlearn one spell every time they gain a level (and replace it, thereby learning 4 new spells that level).
Metamagic Feats
Metamagic feats affect spell level normally. They can be added to a spell as a free action.
Feats
Extra Spell Slot: Prerequisite: 5th level; Give the caster another spell memorization slot. This feat can be taken a maximum of once for every five levels (5th, 10th, 15th, and 20th levels).
Fast Memorization: Reduces the time to memorize a spell into a spell memorization slot to 10 minutes per spell level. Taking this feat a second time reduces this to 5 minutes per spell level.
Improved Mana Recovery: The character gains 15% of their mana after an hour of rest, sleep or meditation. Taking this feat a second time improves this to 20%.
Reduced Mana: Prerequisite: Ability to cast 4th level spells: All spells from one school of magic have their cost to cast reduced as if the spells were one level less (minimum 1 mana point).
[the tables didn't come out perfectly, but it should be readable...]
craftyrat
