Kurogane Hammer is a website created and being developed by Kurogane Hammer, a former global moderator on SmashBoards. It was created on April 22nd, 2015, and has since been accredited by many members of the Smash community as a good resource of technical information.
Its purpose is to record and display frame data and hitbox behaviours for all the characters in Super Smash Bros. 4, as well as most character attributes (such as running speed and dodging frame data), in a way that can be easily understood. While the site was originally for Smash 4 explicitly, it has since expanded to have 3 more games: Super Smash Bros. Melee, Super Smash Bros. Ultimate and Rivals of Aether. Currently, Smash 4 is mostly complete, missing data only for a select few moves, most custom special moves, and some lesser known character attributes like walk acceleration; Melee is a work in progress, Rivals of Aether requires updates from recent patches, and Ultimate is being actively worked on, but there is a large portion of the roster not yet completed.
There are also some humorous references and inside jokes throughout the site; in the character list itself, Lucas is titled "Ridley", Bayonetta's thumbnail is a picture of Masahiro Sakurai wearing Bayonetta's glasses, and Corrin's thumbnail is a picture of Arceus. Some moves and even characters are given nicknames in specific entries, with Little Mac's aerial moves list having the caption "Please don't actually use these", and Kurogane Hammer additionally released a cat version of the website on April Fools' Day, which consisted of thumbnails of cats for every character.