Blogger has that feature. At the bottom of the screen you can either start to type out which category you want the post in, or you can expand the whole list and all of the ones you've created will open and you can click on the one(s) you want. To create a new category you just type it into the box.
I swear it's dead easy. Even I can manage!
Deadra, Wife to Adam , Mama to Beatrix (02/08), Hudson (01/10), and Mazarine (12/13)