What if technology had taken a different path? What if the steam powered technology of the Victorian era had continued on to the present day, offering up the same devices we use but with the metal, gears and wheels prevalent in those early machines? That's the idea behind steampunk, current day tech with a steam era look.
Because of the current high interest in steampunk, quite a few artists and craftsmen have built or modified everything from computers to wristwatches with a vintage sci-fi appearance. They're pretty neat, too, but I haven't seen many motorcycles that would fit the steampunk category, some come close, maybe, but more by chance than purpose.
The Bobster trike shown above has strong hints of it, something I didn't think about until I saw a few others comment and I would have to agree. Brass and copper are a strong design cue. Some point to the Confederate Wraith and when you step back for a second, yep, it has it too.
The Hubbard Steamcycle goes without saying, if you actually use steam power, you have ten points to begin with, add a little copper and brass and you're in.
The interesting machine on the right started as a drawing in a biker comic and the artist's brother thought it was so neat he decided to bring it to life. It doesn't actually run on steam but it looks like it should.
The single engine cylinder is from a tractor and it displaces 1440cc, think about what that sounds like! Tires and wheels are from a Ford Model A. The wooden fork is made of ash with forged iron strapping plus there's a shovel for a seat. Practical? Are you kidding? Cool? Of course.
There are other examples floating around out there, I only tossed out a few, but I think any design that focuses on the mechanical with a few added gears and wheels plus some brass and copper is pretty neat. Steampunk is all about the design and very little concerned with added (or lost) function but some of it would be neat if it worked half as well as you wish it would.