What do you call the son of your mum's cousin? How about the husband of your mother's uncle's daughter? Type how you're related into Wolfram Alpha's computation engine, and it returns the relationship — first cousin once removed, in that last example.
Not only do you get a sketch of the family tree when you enter something like "great-uncle's son's daughter", but you get a quick chart explaining just how related you are to the person you're describing, in a "blood relationship fraction". You may now proceed to whittle down your Christmas card list, as well as impress everyone with your proper genealogical naming at the family reunion.
My Cousin's Cousin's Niece's Grandfather Said to Just Ask Wolfram|Alpha [Wolfram|Alpha Blog]