Regenerate Mana

Diablo II	Notation	Basic	Class	Skill	Item	Ally	Quest	Area	Monster	Tale

All Attributes	Strength	Dexterity	Vitality	Energy	Stamina	Stamina Drain	Heal Stamina
Life	Maximum Life	Life (on Level)	Replenish Life	Open Wounds	Life Stolen	Life After Kill	Stamina damage
Mana	Maximum Mana	Mana (on Level)	Regen Mana	Dmg to Mana	Mana Stolen	Mana After Kill	Mana damage

Regenerate Mana % increases the rate at which a player character regenerates mana:

Mana regenerated per second = 25 * [ [256 * Maximum Mana / (25*120) ] * (100 + Regenerate Mana) /100] /256 [1]

In the absence of any additional Regenerate Mana percentage from the sources listed below, any character will regenerate from zero to maximum mana in ~120 seconds (2 minutes), regardless of maximum mana. However, while a character with 600 mana and another with 1,200 mana will take the same amount of time to regenerate from zero to maximum mana, the former will do so at a rate of 4 _251/256 mana per second while the latter will do so at a rate of 9 _246/256 mana per second.
Ideally mana regeneration should equal mana consumption to allow skills to be used indefinitely, although practically this normally isn't necessary since skill usage is finite, and there are intervals between skill usage which allow mana to regenerate without consumption.
For example, a skill like Tornado with 10 mana cost and 10 frame cast rate consumes 25 (10 * 25/10) mana per second when cast continuously: a Druid with 3,000 maximum mana would regenerate 25 mana per second, allowing him to cast it indefinitely. However, while a Druid with just 1,500 mana would only regenerate 12.5 mana per second, it would take 120 (1500 / (25-12.5) ) seconds or two minutes to run out of mana when casting continuously, which shouldn't normally be necessary for that length of time.
Alternatively, Regenerate Mana 600% would allow the Druid to cast indefinitely with as little as 434 maximum mana:

25	= 25 * [ [256 * Maximum Mana / (25120) ] (100+600) /100] /256
256	= [ [256 * Maximum Mana / 3000] * 700 /100]
{256/7}	= [256 * Maximum Mana / 3000]
Maximum Mana	= {37 * 3000/256}
Maximum Mana	= 434

Shrine

Mana Recharge Shrine applies Regenerate Mana 400% to player who activates it.

Skill

Value	Skill	Target	Min Rlvl
18+(12*slvl)%	Warmth	Self	1
275+(25*slvl)%	Meditation	Self and allied players in (28 + (4*slvl) ) /3 yard radius	24

Helm

Value	Source	Min Rlvl	Ladder
8%	Chipped Skull in helm socket	1	No
8%	Flawed Skull in helm socket	5	No
12%	Skull in helm socket	12	No
12%	Flawless Skull in helm socket	15	No
19%	Perfect Skull in helm socket	18	No
15%	Eth Rune in helm socket	15	No

Value	Source	Min Rlvl	Ladder
(4-10)%	Caster helm	13	No

Pelt

Value	Source	Min Rlvl	Ladder
17%	Aldur's Stony Gaze Hunter's Guise	36	No

Amulet

Value	Source	Min Rlvl	Ladder
(4-10)%	Caster Amulet	13	No
20%	Arcanna's Sign Amulet	15	No
40%	Civerb's Icon Amulet	9	No

Body armor

Value	Source	Min Rlvl	Ladder
8%	Chipped Skull in body armor socket	1	No
8%	Flawed Skull in body armor socket	5	No
12%	Skull in body armor socket	12	No
12%	Flawless Skull in body armor socket	15	No
19%	Perfect Skull in body armor socket	18	No
15%	Eth Rune in body armor socket	15	No

Value	Source	Min Rlvl	Ladder
(4-10)%	Caster body armor	13	No
(10-15)%	Steel Carapace Shadow Plate	66	No
(10-15)%	Ormus' Robes Dusk Shroud	75	No
15%	Stealth body armor	17	No
15%	Bramble body armor	61	No
25%	Heavenly Garb Light Plate	29	No

Melee weapon

Value	Source	Min Rlvl	Ladder
16%	Eternity melee weapon	63	No

Axe

Value	Source	Min Rlvl	Ladder
20%	Skull Splitter Military Pick	21	No
25%	Spellsteel Bearded Axe	39	No

Dagger

Value	Source	Min Rlvl	Ladder
15%	Wizardspike Bone Knife	61	No

Rod

Value	Source	Min Rlvl	Ladder
(4-10)%	Caster rod	13	No

Scepter

Value	Source	Min Rlvl	Ladder
10%	Zakarum's Hand Rune Scepter	37	No
15%	Hand of Blessed Light Divine Scepter	42	No

Staff

Value	Source	Min Rlvl	Ladder
5%	Arcanna's Deathwand War Staff (3 items)	15	No
10%	Mang Song's Lesson Archon Staff	82	Yes (LoD only)
(15-30)%	Obsession staff	69	No (D2R only)
43%	Spire of Lazarus Gnarled Staff	18	No

Wand

Value	Source	Min Rlvl	Ladder
5%	Torch of Iro Wand	5	No

Missile weapon

Value	Source	Min Rlvl	Ladder
20%	Harmony missile weapon	39	Yes (LoD only)

Shield

Value	Source	Min Rlvl	Ladder
15%	Eth Rune in shield socket	15	No

Value	Source	Min Rlvl	Ladder
(4-10)%	Caster shield	13	No
15%	Rhyme shield	29	No
15%	Splendor shield	37	No

Shrunken head

Value	Source	Min Rlvl	Ladder
33%	Homunculus Heirophant Trophy	42	No

Ring

Value	Source	Min Rlvl	Ladder
(4-10)%	Caster Ring	13	No
20%	Manald Heal Ring	15	No

Belt

Value	Source	Min Rlvl	Ladder
(4-10)%	Caster belt	13	No
15%	Gloom's Trap Mesh Belt	36	No
30%	Lenymo Sash	7	No

Boots

Value	Source	Min Rlvl	Ladder
(4-10)%	Caster boots	13	No
10%	Marrowwalk Boneweave Boots	66	No

Gloves

Value	Source	Min Rlvl	Ladder
(4-10)%	Caster gloves	13	No
25%	Magefist Light Gauntlets	23	No

Set bonus

Value	Source	Min Rlvl	Ladder
8%	Angelic Raiment (Full)	12	No
12%	Arcanna's Tricks (3 items)	15	No (D2R only)
15%	Trang-Oul's Avatar (2 items)	49	No
16%	Cathan's Traps (2 items)	11	No (D2R only)
30%	Trang-Oul's Avatar (3 items) ¹	54/62	No
45%	Trang-Oul's Avatar (4 items) ¹	62/65	No
60%	Trang-Oul's Avatar (Full) ¹	65	No

¹ Regenerate Mana 15% is added with each additional item equipped.

Reference

Librarian, mana regeneration (FAQtoids)