I'll go out on the proverbial limb and assume that you're talking about a dovetail jig, Steve.
There are many available - both for use with a hand-held router, and for use on a router table. My preference is for hand-held router use.
I like the Porter Cable 4200 series with the mini template for use with thin stock. It is, however, a fixed-spacing design. If you want variable spacing on through dovetails, consider either the Porter Cable OmniJig, or the Leigh DR4. Leigh also has fixed-spacing jigs. Both companies have been around for a long time, and are likely to remain in business. Other manufacturers seem to come and go.