The mana system was orginally posted by Psychedelic Goblin (always him !), i only included the Mana weather notion.

I came across the deviant Mana system on the crucible archives the other day, and like the 'magic point' idea.
The negative mana idea seemed a bit convoluted though, so I've written my own varient.

Arcane Energy System

Description
Magic energy is measured in points called Arcane Energy.
A spellcaster has a number of Arcane Energy Points (AEP) equal to their Basic Magic Rating (BMR) x 5.
Therefore a spellcaster with a BMR of 6 would have 30 points.
Each spell cast uses 1 AEP per level, so a 10th level Mystic Bolt would cost 10 AEP.
When a spellcaster reaches 0 points of energy they usually rest, regaining energy before casting more spells. However, they may in dire circumstances, cast beyond their tolerance.
In these cases, the excess magic courses through their body like wildfire, rupturing and burning them.
To summon the courage for this act (which is quite obviously, highly excrutiating) they must succeed at a standard WILL roll.
If they succeed they may proceed with the casting.
They choose the level and spell as normal and cast it normally, with normal chances of failure. Then they pay the price, wether they succeeded or not... Each point of AEP summoned does 1HP damage to the caster, plus an extra D6 to the overall total. It is quite possible for the caster to kill themselves when casting a powerful spell after they've expended their original pool of points.
Regaining energy requires rest or meditation (maybe rituals in the case of religious spellcasters). Each hour of rest recuperates 5 AEP.

Example of spellcasting when out of energy:
Sharnas has fought in a mighty battle, having just expended the last of his energy pool on an Arcane Bolt that fried several Beastmen. Unfortunately he is on the losing side, and many more Beastmen are advancing. He notes that his comrades are wounded and need time to escape. He needs to buy them that time...
Summoning his will and determination (he rolls the standard WILL roll and succeeds) he steps forward and casts a level 15 Arcane Bolt! He succeeds in the casting, the bolt arcing from his fingertips as the energy tears through his body!
He feels the blood rupturing from his lips, his flesh searing (he takes 15 +D6 damage from the spell, rolling 5 for a total of 20. Unfortunately he only had 17 Hit points...). he dies in a ball of flaming, crackling energy and melting flesh, his last sight, that of exploding Beastmen and escaping companions...

Mana weather
Some places are full of magic energy, other drains it. Each place has a Magic Aura represented by an Aura level.
For each difference in Aura level the AEP (current and maximum) are increased (or decreased if the difference is negative) by five. The change in AEP takes place when entering a different Aura.
For example Aren, a Cymrillian Mage with 35 current AEP, leaves the Lyceum (Aura level of 3) for the streets of Cymril (Aura level of 1), his AEP are decreased to 25. Two hours later, he is back to the Lyceum with 15 current AEP, his AEP are increased by ten when entering the Arcanum.
The Aura level is normally between -3 and +3, with 0 the average level on Talislanta. The level is higher in places where magic is used constantly (like the Lyceum Arcanum).
Some areas have chaotic mana weather (like in the Wilderlands) the aura level changes with time, sometimes loosing 3 or more levels in a hour or less. Be careful because few people can resist brutal change in Aura level.When the difference is 3 or more, the excess magic courses through the body and the mage should succeed a CON - 5 roll or loose the difference in level x 3 + 1D6 HPs !
I'll put here soon the principal Talislanta Aura levels.