The World's Largest Dungeon
The D20 system is an Open-Gaming License role-playing system from Wizards of the Coast (WoTC). As I do not pretend to have the expertise to correctly explain the license in detail, the basic concept in layman’s terms is that the rules system WoTC produced is free for any game developer to use in commercial products of their own desing as long as they promise to not use or reference anything that WoTC owns as an IP. If the D20 system is used for non-profit (i.e., to make house-rules or systems for personal use) the same rules apply. Furthermore, the 3.0 or 3.5 D&D core rulebooks from WoTC must be required for use with any setting or supplement using the D20 system and the information within the core rulebooks cannot be reproduced.
We are using the 3.5 version of D&D.
The WoTC 3.5 SRD website can be found here: http://www.wizards.com/default.asp?x=d20/article/srd35/.