Sorrry, I am not sure I can give you a complete list of projects, that use the USB Host, but off the top of my head here are a few devices and the like that we have code support for:
Simple HID (Human Interface Device) like Keyboards, Mice, tablets, Joysticks.
Some other joysticks which may not be fully HID
We support some subset of Bluetooth controllers and connections to things like PS3 and PS4 game controllers
USB To Serial devices, like FTDI cables and ones by a few others like Prolific, CH...
Midi controllers
MSC (Mass Storage Controller) - Not in builds yet, but on some threads here. This allows you to plug in Devices like thumb drives, SD Adapters, some hard drives. Currently the code is limited to Fat type file systems (Fat16, Fat32, ExFat). Only up to 4 primary partitions.
I know I am missing some things, but should give you a general idea.