Very easy to make. You can make them with pretty much anything if you aren't a stickler for authenticity.
One of the best flavors I have made was what I called "Spaghetti Onigiri". For that I took a basic American style spaghetti sauce (tomato sauce + ground beef) and simmered it down until it was very concentrated and almost dry. I used that for the filling. Then I took the filled rice ball and rolled it in a mix of grated parmesan cheese, garlic powder, and "Italian seasoning" blend.
Another version I didn't invent is ketchup onigiri. This is made by mixing dried chopped onion and ketchup into cooked rice with small cubes of cheddar cheese. Form it into smallish onigiri and toast in a medium to medium low pan, brushing the outside with soy sauce. I don't remember where I learned it other than some random little Japanese cookbook I picked up Kinokuniya in the late 90s.
A popular standard in my home is to make them with tuna salad. It's probably the most common way we eat them. Just mix your tuna salad to be a bit dryer than usual.
I replied to another thread here with some links to more traditional recipes.