In addition to Gadget Zero (used primarily for testing and development with drivers for usb controller hardware), other gadget drivers exist.
There's an ethernet gadget driver, which implements one of the most useful Communications Device Class (CDC) models. One of the standards for cable modem interoperability even specifies the use of this ethernet model as one of two mandatory options. Gadgets using this code look to a USB host as if they're an Ethernet adapter. It provides access to a network where the gadget's CPU is one host, which could easily be bridging, routing, or firewalling access to other networks. Since some hardware can't fully implement the CDC Ethernet requirements, this driver also implements a "good parts only" subset of CDC Ethernet. (That subset doesn't advertise itself as CDC Ethernet, to avoid creating problems.)
Support for Microsoft's RNDIS protocol has been contributed by Pengutronix and Auerswald GmbH. This is like CDC Ethernet, but it runs on more slightly USB hardware (but less than the CDC subset). However, its main claim to fame is being able to connect directly to recent versions of Windows, using drivers that Microsoft bundles and supports, making it much simpler to network with Windows.
There is also support for user mode gadget drivers, using gadgetfs. This provides a User Mode API that presents each endpoint as a single file descriptor. I/O is done using normal read() and read() calls. Familiar tools like GDB and pthreads can be used to develop and debug user mode drivers, so that once a robust controller driver is available many applications for it won't require new kernel mode software. Linux 2.6 Async I/O (AIO) support is available, so that user mode software can stream data with only slightly more overhead than a kernel driver.
There's a USB Mass Storage class driver, which provides
a different solution for interoperability with systems such
as MS-Windows and MacOS.
That Mass Storage driver uses a
file or block device as backing store for a drive,
The USB host uses the BBB, CB, or CBI versions of the mass
storage class specification, using transparent SCSI commands
to access the data from the backing store.
There's a "serial line" driver, useful for TTY style operation over USB. The latest version of that driver supports CDC ACM style operation, like a USB modem, and so on most hardware it can interoperate easily with MS-Windows. One interesting use of that driver is in boot firmware (like a BIOS), which can sometimes use that model with very small systems without real serial lines.
Support for other kinds of gadget is expected to be developed and contributed over time, as this driver framework evolves.