Coolest capstones?

[deuxhero]

Loremaster has a a crap divination effect that lacks the parts (time and cost) that make it crap 1/day.
Pyrokinetecist, while mostly crap, has an infinite (every other round unless something in splat let's you focus as a swift action) use save or die that (unlike the rest of it's abilities) works on things immune to fire damage. Technically not a capstone, but at level 9 and the real one sucks.

[Littha]

I'm rather fond of the dread necromancer capstone... its unfortunate that the previous 11 levels are pants...

[KellKheraptis]

Incantatrix and Dweomerkeeper have cool capstones.  Aka epic feats pre-epic

[Hijax]

Master of Many forms. at level 10, when you change shape with alter self/polymorph/shapechange, you can add one supernatural or extraordinary ability from any other creature whose form you could assume with the spell, or substitute one of your new form's ability scores for that of any other creature whose form you could assume with the spell.

the catch? lose four levels of spellcasting.

[snakeman830]

Pyrokinetecist, while mostly crap, has an infinite (every other round unless something in splat let's you focus as a swift action) use save or die that (unlike the rest of it's abilities) works on things immune to fire damage. Technically not a capstone, but at level 9 and the real one sucks.

I had great success with a houseruled replacement for Conflaguration called Anarachic Fire.  Pyrokinetecist abilities dealt have fire damage, half chaos damage.  Creatures vulnerable to Fire took double Fire damage (instead of normal *1.5) and Lawful creatures took double damage from the Chaos.  Net result was:
Creature immune to fire: 1/2 damage
Non-Lawful Creature vulnerable to fire: *1.5 damage
Lawful creatures immune to fire: normal damage
Lawful creatures not vulnerable to fire: *1.5 damage
Lawful creatures vulnerable to Fire: *2 damage.

It worked quite well.

[Nunkuruji]

Illithid Savant, I eat your capstone, OMNOMNOM 

[The_Mad_Linguist]

Master of the silent sound (dragon 297).

Looks like a typical "trade four levels of spellcasting for some mostly useless abilities" class.

The capstone ability is a 1/day spell-like ability.
It replicates any spell of 9th level or lower.  As a full round action.  With CL = character level.   

[Aharon]

Walker in the Waste... You don't have to wait for the Capstone quite as long as the Dread Necromancer, and liches always have a place in my heart since BG2 

[Agita]

IotSV deserves mention, I think. Even just visually, Kaleidoscopic Doom is badass.

[Sohala]

Dragon Ascendant for immortality and an awesome aura. Cause really, what is more awesome than making people fear your awesomeness?

[Havok4]

Factotums have an amazing capstone that lets them imitate any ex class feature of a base class for a minute a day, which includes spell casting.

[PhaedrusXY]

I like the Anima Mage capstone: the ability to cast any spell once per day as an Immediate Action. With early entry, you can actually reach that at a reasonable level, also.

[RelentlessImp]

Nightmare Spinner. 3/day make someone die of fright. The DC's kinda crap though, even if you are a Sorc (15+CHA mod).

[PlzBreakMyCampaign]

Master of Many forms. at level 10, when you change shape with alter self/polymorph/shapechange, you can add one supernatural or extraordinary ability from any other creature whose form you could assume with the spell, or substitute one of your new form's ability scores for that of any other creature whose form you could assume with the spell.

the catch? lose four levels of spellcasting.

That's master transmogrifist (spelling?). The real MoMF just gives you agelessness and the shapechanger subtype, along with another wildshaping boost.

[Hijax]

Master of Many forms. at level 10, when you change shape with alter self/polymorph/shapechange, you can add one supernatural or extraordinary ability from any other creature whose form you could assume with the spell, or substitute one of your new form's ability scores for that of any other creature whose form you could assume with the spell.

the catch? lose four levels of spellcasting.

That's master transmogrifist (spelling?). The real MoMF just gives you agelessness and the shapechanger subtype, along with another wildshaping boost.

oh yeah, right. master transmogrifist is the one. It has a special place in my heart, even though power-wise its kinda meh.

[Sinfire Titan]

Incarnate: 1/day, for 3+Wis rounds, I glow bright blue and kick your ass twice as hard while being nigh immortal to noncasters.

Totemist: 1/day, for 1 minute/Con modifier, I glow bright blue and make my awesome class feature twice as potent. What's that? You made the save against my Basilisk Mask's petrification? Try again, and this time the DC is increased by 30%. Sucker.

Warblade: Dual Stance is some good...

Jade Phoenix Mage: Just cause stuff blowing up was awesome enough, we decided to make it even better!

[CantripN]

Truenamer has the BEST Capstone EVER. Crappy class, awesome capstone! Say my Name and I'm There!

[Sinfire Titan]

Truenamer has the BEST Capstone EVER. Crappy class, awesome capstone! Say my Name and I'm There!

"Biggy Smalls..."

[mans0011]

Planeshifter, IIRC, is basically Psionic Genesis. You also get planeshift at will, telepathy, and some resistance/immunity to certain planar effects. It's a cool class but loses a couple caster levels. So, unless someone ever lets me take full progression or I take it in a gestalt game, I'll never play it, lol.

[snakeman830]

Metamind, while still a pretty bad PrC, has one of the coolest capstones out there.  There's a hell of a lot you can do with 10 rounds of infinite power points.  Temporal Reiteration just makes it cheesy.

Cabinet Trickster has a pretty good capstone ability of Detect Thoughts at will and also the Doppleganger's Change Shape ability.  And the Stun thought trick isn't bad either.

Renegade Mastermaker's capstone that makes you a Living Construct is pretty damn cool.