Grunts by Mary Gentle is absolutely brilliant if you like your humour on the dark side. I've read it three times now and it has tears of laughter rolling down my cheeks every time.
The plot is about a group of Orks who keep on ending up on the front lines of the Last Battle, where they get slaughtered and the forces of good always win. That is, until the Orks find a dragon's treasure of modern military equipment that transforms them into super warriors.
As far as I know, this is the only comedy Mary Gentle has done, which is a huge shame as she really has a talent for it. If you like reading Terry Pratchet books, you will love this.