sudo apt-get install nzb
It seems to be lacking some features that I've found useful in command-line programs like Hellanzb — like autounrar and autopar — but it gets the job done.
I'm definitely interested in hearing about any other Ubuntu NZB GUI clients out there, if anyone can give a recommendation.
Happy Mothers Day
To all you people who came from a womb, make sure to give thanks to the holder of the vagina you came out of!