Generic Singlespeed Conversion Kit
This kit converts a standard Shimano-style freehub body for use with a singlespeed cog. It includes a bunch of spacers of different thicknesses so you can get the chainline perfect, as well as a cog and lockring.
We like this kit because it has the widest assortment of spacers of any we've found. Please note the included cog (like any cog included in a spacer kit) has a thin base and isn't the best quality. We always recommend using a Surly cog for this purpose.