Woodstock International Half-Blind Dovetail Jig W1099
This jig runs between $40 and $60. It works pretty well and was one of my first jigs.
A similar jig is sold by Harbor Freight under the name Central Machine and though it LOOKS like the same tool it is inferrior in many ways including fit and finish. I suggest that you not pay more than $29 for this jig, so don't get too bid-happy on eBay!
Click this link for Woodstock and Central Machine Dovetail Jigs for Sale on eBay (For as many for as many of these that have been sold there is surprisingly light activity)