Welcome to my Big Bad List of Roleplaying System Reference Documents, a gateway to open rule systems in a handy hyperlinked format. If you know of any I haven’t yet listed please let me know.
The Hypertext D20 SRD (D&D 3rd Ed)
D20 Heros SRD (Mutants and Masterminds)
Basic Role Playing
Mudge, Mook’s Fudge (Hypertext rules, not specifically an SRD)
Fate Roleplaying Game SRD (Core, Accelerated Edition, and Toolkit)