The Hypertext D20 System Reference Documents is an excellent resource if you’re not already familiar with it. It presents the System Reference Documents in a convenient, easy-to-read, hyperlinked format. This includes all of the D&D core rules minus any Wizards of the Coast closed content - the character creation rules, for example, and a few of the monsters. While the SRDs are not a complete replacement for the core rulebooks (it lacks the advice given in the Dungeon Master’s Guide), it is excellent for quickly looking up rules, monsters, spells, magic items and the like.
As a bonus, the site comes with various extras, including the more or less complete Epic rules (Epic Level Handbook), Psionics rules (Expanded Psionics Handbook), Divine rules (Deities & Demigods) and Variant rules (Unearthed Arcana) and extras such as character record sheets. The variant rules alone are an excellent read for any DM. This page is highly recommended, and absolutely invaluable if you run D&D online or use a computer when running D&D.