Hi yes hello you have my attention, here are some things I had on hand as links. Some of these touch on things mentioned on this iceberg, some don't. I will try not to go on at too much length.
SOF Soundtrack. Released in a few different ways, all of which were free, but some required entering a contest. Had to change the download link, let me know if it's not working.
The Concept Farm. Ad agency that did a bunch of fun alternate-reality-style ads, and was responsible for writing at least the Blogspot edition of Fang's blog.
Videos/trailers. Most are on YouTube, but the very first one on that page was a RealMedia file I had to convert to mp4.
A little bit about ghost-writing. Gabrielle Charbonnet certainly worked on the first four books, and likely continued working on the series thereafter, but IDK for sure. Like she definitely worked on HAWK but other than that I can't be sure.
Various ephemera. In this tag on this blog, and this one on my old URL, there's a random scattering of various things. Interviews. Screencaps of old Flash games. Things I saw on Ebay, including Advance Reading Copies. Some "leaked" pages from the third book that were posted on the blog. Pics from some of the apps that existed. When I get around to it someday, this stuff will end up on this page.
There is a "secret chapter" of additional material that only appears in full in certain editions of the first book.
Something similar appears in the form of material related to ITEX, released on the website and also in certain editions of the second book, as part of the marketing for the third book. (There is something of a story about Roland ter Borcht that runs through these snippets of fake newspaper articles and ticket stubs.)
There is a skeletal list of official sites here. Any of the old official websites could fuel an entire iceberg of its own. Maximum-X, later Max-Dan-Wiz, is the best archived and I believe it had the highest population. MaximumRide.COM was the very first official website, and had its own dedicated forum, but it's all poorly preserved. Maximum Ride.CO.UK was somewhat popular in the time between MR.COM going down and MX coming online.
Reading guides. At least six of the books had reading guides composed for them, intended to be used by educators.
Concept art for the FANG trailer can be found here.
A comprehensive index of Fang's blog can be found here. Organized by topic, then chronologically within each topic.
A purely chronological index of Fang's blog (and other character blogs) as seen on MX/MDW is here.
Max's Tumblr account is still online here. Ran for just a few months in 2012, from March 29 through August 5.
Fanfiction.net's section for Maximum Ride was an extremely active place between at least 2005 and 2011, which is when my own activity there began to taper off. It's been a long time and my memory isn't getting any better, but I recall PhoenixFanatic and StFangOfBoredom (old tumblr account) being well-known writers at different times. If you can get the website to load, sort for "most reviewed" fics and you should get at least a hint of what was best-known.
The Cafepress store functioned for almost 20 years before beginning to crumble. You should still be able to view the designs here.
There were a couple of flash games over the years. There was a platformer-type game on MaximumRide.COM at one time. Something different on MRUK that I don't remember well.
The last thing I'll mention is that there was a very active Maximum Ride/Maximum Ride-adjacent roleplaying scene, beginning on MaximumRide.COM and continuing through the end of MDW. Not always the same people involved, but anywhere Maximum Ride got discussed, someone would be interested in roleplaying there. I've never been a roleplayer so I don't really know what they got up to, but I know it was a lively sub-community.
That's all I could dredge out of my brain for the time being.