RPGGeek's database entry for the d20 Modern family of games lists 13 full-fledged games that are adaptations of d20 Modern, and the entry for the d20 Modern game itself lists 7 settings (often a good source of new rules), one 3rd-party quickstart, and over 500 supplements with new or alternate rules.
The entry for the d20 family of games is enormous, being over 100 games and 30+ settings. The list of materials linked under just the 3.5-compatible d20 System itself numbers over 2300 items, some of which are complete overhauls of the SRD and are really games unto themselves. Like the 3.5e d20 SRD entry, each of the other games listed under the d20 family entry also has its own list of supplemental material. You could spend months (or years) just investigating the discoveries made by drilling down through the connections under the d20 top-level database entry.
Many of these are going to be commercial products simply because of the sheer volume of commercial activity that surrounded and still surrounds d20, but RPGGeek's criteria for listing is merely "published", so many of those will be fan and amateur publications as well.