Washed Pit Sand or USGA Sand
If you have a particularly fine lawn and use a cylinder mower then this is what you need and this product is used by Bowling and Golf Clubs alike. USGA Sand is (United States PGA certified sand) specified for use on Golf Courses around the world.
Medium Bedding Sands
If you have a lawn laid on a very heavy clay and you are trying to improve the drainage, then either of these two will help. The best way to start is through lawn aeration (punching hundreds of holes in the surface) and then top dress, letting the coarser sands particles drop into the many holes, thus improving the drainage of the area.