This tradition has made a big revival in the past few years and one can see this cake in many weddings nowadays.
The question of course comes up: what kind of cake should you have? As the tradition might indicate, the grooms cake was centered around the bridegroom.
Thus, it usually incorporated elements of things that he might like.
It was centered around 'masculine' themes, such as hunting, sports, etc.
A quick look on Google for groom's cake will reveal dozens of pictures.
As you can figure out from these pictures, the cake is quite quirky and has none of the 'stacked elegance' of the main bridal cake.
In fact, this is how it is meant to be - a cake with which you can have some fun and let your hair down, instead of having to adhere to the idea of pristine whiteness, as with the bridal cake.
As far as picking the theme is concerned, you, as the bride would know best.
You know best what your to-be husband likes the most.
Does he like sports? If yes, which one? If he is a football fan, why not have a cake with his favorite team's logo on it.
If he was a quarterback in high school, how about one with a football on it? If your guy was/is in a rock-band, a guitar shaped cake would really make his day.
If he likes hunting, why not one shaped like a hunting trophy? If he's into bodybuilding, a dumbbell shaped cake would be great.
The ideas you will get will be unique to your to-be husband.
You can take some hints from guidebooks and websites, but at the end of the day, it is something you have to decide for yourself to make the groom's cake all about your future husband.