The Tempest is a solid enhancement that many current builds are based around. It may not be especially flashy, but has solid passive bonuses. That theme continues with the advanced tiers of the prestige enhancement.
Here's the whirling storm of blades, the Tempest:
Ranger Tempest I
Prerequisites: Ranger level 6, Dodge, Mobility, Spring Attack, Two Weapon Fighting, Improved Two Weapon Fighting
Cost: 4 AP
Benefit: Your training has turned you into a whirling tempest of steel, granting a 10% competence bonus to dual wield attack speed and a +2 shield bonus to armor class when two weapon fighting.
The bonus to armor class now also actually only applies when you're dual wielding. Both of the bonuses granted by Tempest I have been "typed", to clarify stacking questions that constantly arose.
Ranger Tempest II
Prerequisites: Ranger level 12, Tempest I, Ranger Dexterity II, Greater Two Weapon Fighting
Cost: 2 AP
Benefit: Your training has turned you into a whirling tempest of steel, granting a 10% competence bonus to dual wield attack speed, a +3 shield bonus to armor class when two weapon fighting, and your penalty to hit has been reduced by 1.
Since the initial prerequisites for Tempest I are pretty steep, we made the additional prerequisites for Tempest II (and III) on the cheap side. Remember that Rangers get Greater Two Weapon Fighting for free.
Ranger Tempest III
Prerequisites: Ranger level 18, Tempest II, and any one of: Two Weapon Blocking, Two Weapon Defense, Oversized Two Weapon Fighting, or Lightning Reflexes.
Cost: 2 AP
Benefit: Your skill with two weapons is unequaled - your training has turned you into a whirling tempest of steel, granting a 10% competence bonus to dual wield attack speed, a +4 shield bonus to armor class when two weapon fighting, your penalty to hit has been reduced by 2, and you gain additional attacks when two weapon fighting.
The additional attacks gained at tier III are identical to what we originally had planned for (the not-in-the-game) Superior Two Weapon Fighting.
Edit: Added AP costs. Oops.