Spellsurge

Spellsurge permanently enchants a melee weapon to have a 3% chance on spell cast to restore 100 mana to all party members over 10 seconds.

It is taught by Formula: Enchant Weapon - Spellsurge, which is a random world drop in Outland. Requires skill level of 60 in Outland enchanting.

The tooltip on the enchant says the proc rate is 3% however there is much debate about this. In game tests seem to show that the proc rate is actually 15%. It is interesting to note that once the enchant triggers its ability, it also activates a 30-second hidden cooldown during which time the Spellsurge effect cannot trigger again. There is at least anecdotal evidence to suggest that the internal cooldown was raised to 50 seconds in patch 2.1

Along with Major Healing (weapon enchant), Spellsurge is generally considered to be the best weapon enchant for classes in a healing role, particularly in a raid environment.

This enchant is, in the extreme case, an average of 13.56 mana/5s for the caster's group. This assumes chain casting instant spells as soon as the hidden cooldown is over (13 m/5 if chain casting 1.5s cast time spells, since it triggers 1.5s later). In practice, you can expect to get less than that, possibly much less depending on your casting rhythm. Also, for shorter fights the proc timing will be much more variable.