Second Founding
The Second Founding happened in 021.M31. Unlike every other founding since, new Marines were not created during this founding. Instead, the remaining loyal Space Marine Legions were divided into smaller Chapters, to fit with the restrictions of the new Codex Astartes, created by the Ultramarines' Primarch, Roboute Guilliman. Because the Marines that made up each of the Second Founding Chapters originated from one of the Legions that first served the Emperor, the chapters that were created were called Successor Chapters, a title not given to chapters created after that initial split.
Due to the flexible size of the Space Marine Legions and loss or wilful destruction of records in the wake of the immediately prior war it is unclear just how many Successor Chapters were created. Some Legions, such as the Ultramarines, produced vast numbers, where as others produced relatively few. The Salamanders, consisting of only 7 Companies at the end of the Heresy, produced no recorded successors in the second founding at all. It is possible that certain Chapters that are not commonly attributed as descendents of any of the Space Marine Legions were created in the Second Founding.
Some sources have placed the founding of the Grey Knights at the time of the second founding and, in an unprecedented action, they were created using gene-seed taken directly from the Emperor himself, rather than from a Primarch or a redevelopment. The facts surrounding this theory are unclear though, as it would seem unlikely that the Emperor would be willing to order the creation of such a chapter, or offer of his own flesh for the gene-seed, whilst in his incapacitated state in the Golden Throne. In either circumstance the Grey Knights are not considered part of the Second Founding.
Some Reputable Second Founding Chapters