Here's a bit of an unsung hero in my shop, the tapered tenon rounder.
Usually, I try to make all of my tapers on the lathe, where they are done quickly and accurately. But on occasion, especially if the piece is already bent before shaping the tenon, I pull out my rounder. Elia Bizzarri sells a version of this tool, that while adjusted differently, cuts a beautiful tenon. You can see it on his sight www.handtoolwoodworking.com.
What I like about my rounder, besides being cheap (you do have an old frog laying around somewhere, don't you?), is that it's as adjustable as a handplane. The control that I have over the adjustments overcomes my usual hesitance to use and set up a jig. And the results are dead on. As with most of my jigs, I found a rare piece of water stained poplar laying around the shop for this one, I guess I'll have to find another use for all that exotic hardwood.
It's simple to make. First drill a hole in a block and ream it with the reamer that you are matching. Then, cut the top off the block close to the hole. I like to finish off with a handplane until I get a nice even slot along the top of the hole. Then cut a separate bed for the frog you are using and screw it to the block. Mount the frog and you're just about there.
I like to curve the blade a bit where the tenon enters, this help shear excess material down to the size that will fit in the hole. When making a tenon, I still find it helpful to rough shape it before using the rounder, this helps to ensure that the tenon is centered properly and saves wear and tear on the tool and my wrists! (quick tip: wear those rubber dipped gloves from the hardware store, they'll save your joints)
As you can see in the image, once the tenon exits the rounder, it becomes a straight tenon that equals the size at the end of the tapered hole, with this in mind, you can easily modify this design to make straight tenons as well.