Facom Pin Wrench

The Facom 117.B is one of the finest adjustable pin wrenchs I have ever seen. It comes with four sets of interchangable pins, each with two different size options. The position of the spanner is held fixed via a quality play free turnbuckle. It ain't cheap, but it works great. Commonly found for around $65.

For many single speed bicycle eccentric applications, a PARK #SPA-1 Pin Spanner: Green can be used instead, if money or space is tight. It costs about $8 online.