Does anybody know any animes with really cool transformations?

Do you mean an anime where the protagonist or antagonist has a really cool transformation? Well, I do recommend The Girl Who Leapt Through Time. It has nice animation and a good ride through the past, present and future. Here are some others I recommend:

Sailor Moon
Card Captor Sakura
Full Moon o Sagashite
Howl's Moving Castle
Figure 17
Magical Meow Meow Taruto
Mai-HIME
Shugo Chara!
Ponyo On the Cliff By The Sea
The Cat Returns
A Little Snow Fairy Sugar
Fruits Basket
Moon Phase

DRAGON BALL Z!!! there great apes super saiyan and fusions!!!!

Tokyo Mew Mew has a bunch of girls who transform into Mew Mews. In this Video the team of Mew Mews all transform and use their attacks!

DRAGONBALL Z
SSJ,SSJ2,SSJ3 AND SSG

Eren from Attack on Titan and transform into a Titan, so that's pretty cool

pretty cure
shugo chara
sailormoon
dragonballz
mermaid melody
Cardcaptor Sakura
Mythical Detective Loki Ragnarok(not really transforming)
Petite Princess Yucie

The only anime I've seen with a transformation scene is ecchi, but if that doesn't bother you, Panty and Stocking with Garterbelt has two transformation scenes.

hmm most of them where already mentioned, but maybe: kamikaze kaitou jeanne or tokyo mew mew...

Revolutionary Girl Utena, Sailor Moon, Wedding Peach, Tokyo Mew Mew, Magic Knight Rayearth, and others!

Um... id have to say these do:
Ranma 1/2(i guess u can call it a transformation)
salior moon
natsume yuujin chou
soul eater
Fruits Basket(sorta)
Shugo Chara
and Tokyo mew mew


o.k before you see any of these um.. ranma 1/2 looks like this (he transforms into the girl)

Um,I'm not sure if this counts but,I posted this anyway of Erza after she reequips to another armor..

naruto
sasuke
winx club
dragon ball z
cardcapture sakura
vampire knights

I only know DragonBall Z/GT and Bleach. I prefer in DBZ transformations of super saiyans. There are 3 forms in Z and Gt has one transformation more: Super saiyan, super saiyan 2 and super saiyan 3 (Super saiyan 4 in Dragonball GT). Then there is Oozaru transformation (A giant ape).
But I like more Bleach cause there characters wont change but swords instead changes. Here is a video where are few bankais. :3

Yoroiden Samurai Troopers (or Ronin Warriors)
Guyver the Bioboosted Armor
Dance in the Vampire Bund
Rosario + Vampire
Trinity Blood

precure!!

There is also Kill La Kill
its pretty epic and was made by the same two guys that created Gurren Lagann another epic anime everyone should watch if they haven't already
In Kill La Kill there are tons of transformations like the elite four's 3 star mk.1,2 and 3 transformations
Ryuko Matoi's four or five Senketsu transformations
Satsuki Kiryuin also has some of Kamui transformations
as well as a bunch of random lesser characters having 1 or 2 star transformations
and heres a video of three of the transformations *mild spolier alert

Katekyoushi Hitman: Reborn

Bro, if you're looking for transformations,
Look at the big three
Dragon Ball Z
One Piece and Naruto, they have some awesome transformations.:)

DRAGON BALL Z RULES!!!