Classes.html allows you to quickly browse each of the official classes in 5e. You can also filter for the class' specific subclasses for easier reference, sharing produced URL for quick-access.
Bestiary.html provides a searchable, filterable list of the more than 760 stat blocks in D&D 5e. You can display them and each attack and damage roll can be done in-line with just a click.
Items.html lists the mundane and magical items currently in D&D 5e, allowing quick access to their properties.
Backgrounds.html lists the official backgrounds in D&D 5e, including those from supplement material.
Feats.html provides a quick-use list of the feats in D&D 5e, including those from Unearthed Arcana and supplements.
Races.html lists each of the races in D&D 5e, including subraces and variants.
Spells.html lists all of the official spells in D&D 5e. The list can be easily searched and filtered, and the entries can be shared through the generated URL.
CRcalculator.html provides a verbose, in-depth Challenge Rating calculator, for determining the CR of custom monsters made for D&D 5e.
You may be interested in 5etoolsR20! This userscript allows you to import much of 5etools' data without paying a dime.