I have to disagree with the writer of the previous writeup. The d20 system is most certainly open. The restrictions that they place upon it are a lot less harsh than some open source licenses.
The restriction is only on putting the d20 logo in your product. If you wish to use any material that has been released as "Open Gaming Content" then you need not do anything at all. You just may not put the d20 logo anywhere in your product. In addition, you may not put any sort of trademark of Wizards of the Coast in your product, as well. (A company needs to "OK" something that actually uses their trademarked names).
So, feel free to go out and make a game which uses the entire d20 system. But, if you want to call it a "d20 game" you must not break any of the restrictions. These include small things like "Include this license with your game, so that other people may derive from YOUR work" and the one that I have a problem with "Do not cover things such as character generation" (Which is WotC's method of selling the core rulebooks which are the only things that may detail those pieces of game info.)
There have been a lot of new d20 games released recently. A super hero game, a swashbuckling game, a lot of game conversions including an official Call of Cthulhu conversion, and a lot of others. What I find exciting is that they are releasing a lot of modules for D&D that are d20 -- so WotC doesn't even have to approve them. This includes the Adventurer's Keep modules by Alderac Entertainment Group. (Cannot find a better price -- $2.50 for a complete module.)
For more information regarding what the d20 license does and does not cover, please check the "d20 system reference document", better known as "The SRD" (http://www.opengamingfoundation.org/srd.html) The open gaming foundation also hosts another game which is COMPLETELY open called "Dominion." (I've glanced at it, and it also looks nice.) Also, wizards has some information about it available at http://www.wizards.com/D20/main.asp?x=welcome,3 (which may change soon, knowing how often Wizards will change the URL of their webpages.)