A slangy way of expressing elation or extreme joy. Literally means "it has arrived".
It's often used on Japanese Internet. It plays a prominent role in Densha Otoko, since it's the first thing you see in the series (Densha otoko kitaaa / Densha Otoko is here / Densha Otoko is awesome).
Two of the main characters in Kamen Rider Fourze also use it frequently. One of the series' catchphrases is "Space kitaaa!!!"