This is a terrific idea, I'm so surprised that gamers, a community famous for its rules lawyers, hasn't done this yet.
"Every dollar spent in a market benefits the market leader, even if they don't receive that dollar directly" When 3e was published, the intention was to reinvigorate the industry with a common ruleset that would de facto require purchase of the core ruleset. This makes D&D the "lingo franco" of RPGs. If you want to publish a game, you may use the SRD. Take "Talislanta" for example. That old, classic RPG reprinted and used the d20 rules, when it already had a developed (albeit intentionally simple) rule system.
The original concept of OGC (Open Game Content) for D&D was that third party publishers would add to and modify the rules, and that other publishers could pick up those rules and use them. In due course of time, the rules that were superior would end up changing the game. E.G. The ENWorld Grapple rules, which are a clear improvement.
However, it's never really gotten past the first step. Publishers seem scared to use each other's rules, even when they are better. We do see compilations, but we don't see a lot of publishers using each other's material.
Now, according to the rules, you can do this:
1.) You can make a program or use a wiki to compile OGC material, the trick being that whomever is entering that material needs to understand how to separate OGC from PI (Product Indentity) If you enter a spell of Relics and Rituals, you can enter the spell stats and the description of the actual mechanics, but not the flavor text which explains the spell's history and common use.
2.) The second problem is that there is no feedback system for this. Netbooks have often used some sort of judging of the material on a variety of criteria. This would need to happen here as well, to guide users to the best material.
3.) It would have to exist with the understanding that some folks may be unhappy you are publishing their rules. Any d20 publisher knows their OGC may be reproduced, that comes standard with the d20 industry. If you try and track down every publisher and get permission for this sort of project, all your resources and time will be tied up in asking permission. I personally always ask a publisher if I can use their rules in a project, but that's individual. Ultimately, you are doing this because you can.
I think this is a worthy cause and well worth persuing. Alas, I can't help other than contribute the occasional bit. I work 14-16 hour days between work, more work, and grad school and I keep my weekends free for kids and homework. So someone else would have to get it started.
I would really like to see something like this. It's a fine idea and long, long overdue.