I used BOOM! in one of my chapters in a paragraph by itself. It was a deliberately slow-moving, thought-filled church setting, and I wanted to startle the character and reader.
I also created spluck! as the sound someone's body makes as the character goes flying into mud. I explain it once, then use it several times thereafter for humorous effect. It's kind of a cross between splash and muck. I'm open to suggestions on that one because it doesn't really convey the great force with which the character hits the mud, in my opinion.
I also use um and er when a character is struggling for something to say.