Copper Harbor: http://www.youtube.com/watch?v=rN9btRzqsvA
Marquette's got a strong mountain-biking scene as well.
Grand Island is a nontechnical loop on what used to be double-track, that circumnavigates the island. Views are nice, and the North Beach makes for an excellent stop-and-rest point.
Bruno's Run is about 15 miles south of Wetmore. It's rooty and rough, which makes it fun in my book.
Ishpemming area has some trails, and also a well-regarded, Wednesday evening ride.
There is probably more out there that I am not familiar with. Generally speaking, most of the mountain-biking goodness that I hear about is in the swath from Copper Harbor down through Ishpeming and Negaunee, and into Marquette.