The Second Founding, of Space Marine Chapters, occurred seven years after the Horus Heresy, in 021.M31. This Founding involved the division of the existing Space Marine Legions into the smaller, autonomous Chapters into which the Space Marines are now organized - as decreed by the new Codex Astartes, created by the Ultramarines' Primarch, Roboute Guilliman.
The remaining loyal Legions (sometimes referred to as the “Progenitor”) were divided into smaller Chapters - one chapter maintained their parent Legion's original name, symbol and colors, while the remaining chapters took new names and heraldry. These remaining Chapters are known as the Second Founding Chapters. Each Second Founding Chapter initially shared their parent Legion's gene-seed - subsequently each chapter's gene-seed was isolated, forming new genetic lines and forged their own histories and traditions distinct from their Progenitors, while still remaining proud of their roots.
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.
It is unknown how many Successor Chapters were created - exactly how many Chapters were founded and from which Legions is unclear. The Apocrypha of Davio attempted to categorize them all, but was never completed. Some survive to this day, the proud inheritors of their Legion's histories and the traditions of their Primarch. One of the earliest known accounts is found within a version of the Codex Astartes known as the Apocrypha of Skaros claiming that the Ultramarines Legion sired twenty-three Primogenitor Chapters, but omitted to list them all. Furthermore, no version of the codex lists a Second Founding as having been raised from the Salamanders Chapter, though this does not mean that none ever were. Due to the scarcity of information regarding the identity and status of the Second Founding Chapters, a huge amount of mystery and legend surrounds many of them. Some Chapters that believe themselves Second Founding may not be at all, while others may be Second Founding but be ignorant of the fact. 
The vast majority of the Second Founding Chapters were created from the ranks of the Ultramarines Legion; Other Legions were not large enough to be divided more than once. The Salamanders, consisting of only 7 Companies at the end of the Heresy, produced no recorded successors in the second founding at all. The division of the Space Wolves Legion produced only one additional chapter known as the Wolf Brothers. It is possible that certain Chapters that are not commonly attributed as descendants of any of the Space Marine Legions were created in the Second Founding.
Some sources (such as 2nd edition's Ultramarines codex) have stated that the Grey Knights were the first of chapters created during the Second Founding. They are an exception among the other second founding chapters in that they have no antecedents, their gene-seed engineered from unknown sources. Their creation is unrecorded, and even their inclusion in the second founding is debatable.
List of Second Founding Chapters
Below is a list of confirmed 2nd Founding chapters with the sources.