Sette Single Speed Conversion Kit
Comes with 16 and 18 tooth cogs, you have the choice in what gear ratio suits your neck of the woods the best. It comes complete with two spacers, 2 cogs, and a lockring. Weighing in at only 68 grams for the 16 T and 85g for the 18 T, the Sette Single Speed Conversion Kit is a weight watcher's dream. For best performance, you will want to use an 8 speed or Single Speed Specific Chain.
This was a good start when I converted to SS. The spacers are definitely sufficient and the cogs are well constructed and can give you a good idea of what size you want. However, I wanted a wider cog base (like Surly makes) as to not strip out my free hub body.
I have had this installed on my back-up bike for 3 years. Used on and off road. It has held up very well, had to use a couple different spacers to get the proper alignment.