Extreme Explorer — Kind of like being Indiana Jones, really.
Archmage — is able to sacrifice high-level spell slots to get things like six fireball spells per day 2 Time Stops (9th level spell) a day at the cost of a 9th level slot (and it can be done 5 times).
It also makes blasting a bit more viable. Imagine what it does with the actually worthwhile spells...
Risen Martyr — "Hey look, I died, and now I can do all this awesome shit!"
Wait, SERIOUSLY? D includes a prestige class that makes you JESUS? HOLY SHIT!
That one was in the book that included the 'saint' template, after all...
Increasing in level in any class arguably makes one more of a badass (that 1st level wizard having troubles fighting kobolds? He's having troubles fighting Pit Fiends now). It's just that the badassness is split out over twenty levels, which generally makes the shift somewhat gradual.
Why there is no mention of Incantatrix, Planar Shepherd, Dweomerkeeper, Ruby Knight Vindicator, Jade Phoenix Mage...? Even if they grant a level in badass to already badass classes While on that, why not add Survivor to the mix?
Planar Shepherd deserves special mention for being the only prestige class that's strictly better than just taking more Druid levels.
If none of the official classes are badass enough for you, try these fan-made rules.
4th edition D&D goes so far as to build this into the classes twice over. First, at level 11, you get a Paragon Path that lets you take a level in badass... then, at level 21, you get an Epic Destiny, which is a second level in even more badass. Epic Destinies include the likes of Master of Moments (you gain control over time), Undying Warrior (there is nothing in the multiverse that can kill you), Parable (you realise that all of reality is but a story... and you ascend to become its author), World Tree Guardian (you are responsible for protecting the spirit of the world itself from harm), Chosen (your god makes you it's avatar within the world), and Demigod (you become a lesser god).
Any RPG character will level up through the course of the game.
If you're playing Deadlands, death can be step one to taking that level. Just ask Abe Lincoln who came back from the dead to run the Union's monster-hunting agency.
The Imperial Guard took a whole bunch of levels in bad ass this year after they got a new codex that's actually good. Also an example of Throw the Dog a Bone since IG and IG players were Games Workshop's favorite Butt Monkey.
Space Marines also took a few levels in badass with their Fifth Edition Codex, the most notable being the Captain's Weapon Skill boost and the Chapter Master's Call Orbital Bombardment ability.
The Eldar got a significant boost with the rules changes in the Fifth Edition rulebook that combine with existing powers to make their Falcons tougher than Land Raiders.
The Solars of Exalted took a collective level in Badass when the Jade Prison broke. For better than fifteen centuries, spanning the Shogunate, through the Scarlet Empress's reign, there were roughly twenty Solar Exaltations active in the world at any given time. Two years after the Scarlet Empress disappeared, that number shot up to one hundred and fifty. All of a sudden, the idea of a Solar Resurgence isn't that crazy, even in the face of the Wyld Hunt.
Likewise considering that any kind of Exaltation is pretty much the epitome of taking a level in Badass. This troper is surprised the trope hasn't been renamed, or at least that Exalted didn't already have an entry.
The Green Sun Princes is just like Abyssal, except it's "go make Creation a living hell". It's an even more (green-)shining example of taking levels in badass compared to the Solars. While Solars exalted by proving their badassitude, the Infernals are exalted by failing to prove their badassitude. They sucked so much they had to make a Deal with the Devil to take levels in badass.
Infernals get to take another level with the Faustian Rebellion Heresy charms. Want to prove your awesome in a way no other Exalt can? Take the power the Yozis have given you, then use it to break, rewrite or cheat every control measure they've loaded on, then build a charmset that embodies You As A Primordial in the same way that the Malfeas charmset embodies Malfeas, then start spawning your own Third Circle Demons.