This page has information on exotic, hard to find, adapters for computers. Due to sloppy use of language, if you want an adapter that goes in the opposite direction from the most common adapters, you will have a very hard time searching for it. People will advertise the same adapter as a USB-to-ethernet and an ethernet-to-USB, for example. Thousands of people pollute ebay and the search engines with incorrect terminology.
The USB-IF failed to standardize terminology, develop standards for opposite direction devices, and doesn't even have product categories for them in their product listings, and don't even list non-certified devices (which is likely for exotic devices). Their "other" category is overflowing (it refuses to display them all) with mundane devices and it doesn't even have descriptions.
These adapters do exist, though very rare and may have limited software support. Some application specific ones exist as well such as a print server which lets you connect a USB printer. The idea is to be able to add a remote USB port that can be accessed, or even shared, by PCs over ethernet. One use is to overcome cable length limitations.
These are needed to allow an old laptop with a 16 bit ISA style 5V PCMCIA 2.1 slot to use USB devices. There are still a lot of uses for an old cheap expedable laptop but sometimes you need to use USB devices and don't need high speed. They can be used as front ends for embedded development, CNC controllers, serial terminals, etc. For example, they are frequently used as serial terminals to connect to the debugger on a microcontroller board. Thing is, many microcontrollers are now coming with USB instead of serial ports, though they may emulate a USB to serial adapter.
Could be made with a SPI-to-USB adapter chip (maxim, microchip, etc). Lots of uses for those, since many PDAs lack USB. Tricky mechanically, since you don't want the USB cable to stress the card. One of the more boring applications would be to use it to read a USB flash drive, when someone hands you one on the go (good idea to make your own USB flash drives from SD cards using a tiny adapter, for just this reason). Might be called a SDIO USB HOST Adapter. If you have a compact flash slot, those adapters are available.
Ok, ones that actually work well aren't that ordinary. They all screw up XON/XOFF flow control when talking to devices with small buffers or which can't handle characters from the serial port while doing something like burning flash. Ones that support the USB communications device class standard are a bit less ordinary.
Serial is too slow to control a USB port? Sure for many applications, but not for others. HID devices are typically slow. USB-to-serial adapters are typically slow. Now why would you want a serial-to-USB adapter to talk to a USB-to-serial? Well, what if the USB-to-serial part is embedded. You have a device that is designed (microcontroller, cell phone, modem with controller, GPS, etc) to plug into a USB port and look like a serial port to the host PC. But what if the Host PC is an old laptop with no USB or isn't a PC at all, such as a port on a terminal server or a dialup modem. Or lets say you have a data logger that emulates a serial port via USB that is in a remote location with no PC and no internet access. Plug it into an old fashioned modem using a serial to USB. Or suppose you have a RS-232 device but you need to use a cell phone to connect to the outside world; with a serial to USB adapter, you can connect such a device to a cell phone. Amateur radio TNCs have serial ports as well. You might also want to connect to a zigbee modem. So, there are two modes, in the same or different adapters, one which emulates a standard serial port and one which allows you to send arbitrary USB commands. A flash microcontroller with a switchable host/slave port or a host port could be used to make a two chip version of this.
Such an adapter would have an added bonus. It would provide for USB slave devices what a USB-to-serial adapter does for hosts. It would allow one to add a serial port with no hardware modifications, only firmware changes. The usb consortium folks defined a standard for sending TTL serial over the USB connector (which requires hardware changes to a lot of transceiver chips or using multiplexors) instead of just defining a standard inexpensive converter chip to do that job and many other.
An AT90USB1287 or AT90USB647 ($11.87 qty 1 digikey) 8 bit AVR flash micro with USB on-the-go could provide a two chip solution (second chip provides RS-232 levels) or one chip for ttl level serial. And as a bonus, it could function as either direction of serial converter. Or the new AVR32 chip when it starts shipping, though that is overkill. And the USB serial device driver from FreeBSD could provide some of the software.
Some companies such as byterunner, B&B electronics, and black box sell USB to dual (or more) serial adapters
Some companies such as byterunner, B&B electronics, and black box sell these
Some companies such as byterunner, B&B electronics, and black box sell these
B&B, among others, sell these, including a 50 meter CAT5 and a 500 Meter fiber optic.
The FTDI chips, or standard modules based on those chips, are frequently used for this application. They are not USB communications class compliant but they are well documented and have Linux support. yliu73 on ebay is one supplier. Some others are sold on FTDI's web store
Some companies such as black box sell these.
This device lets you plug a dual mode USB and PS/2 keyboard into a PS2 port. No electronics, just changes the wires. The electronics is already in the keyboard. Replaces the one that came with the keyboard.
This device would let you connect a USB only keyboard or mouse into an old PS2 port. You might need this, for example, if you are connecting an old device to a KVM switch. And not everything which has a PS2 port is a PC either. Except for cases like a KVM, buying an old keyboard/mouse is likely cheaper.
In the early days of USB, a lot of parallel port devices were interfaced with bridge chips to make USB models (not compliant with standard classes). But the standard doesn't make provision for bit-banging the I/O pins on a standard USB-to-parallel adapter. That was stupid. It needed to be part of the standard protocol and API.
One version of this only supports mass storage devices. Plug it into a IDE port on a PC and now you can connect USB hard drives, USB flash drives, and USB cameras. Another version would simply exploit the availibility of a IDE port to add full speed USB or better functionality (with a special device driver) to a device which otherwise doesn't support USB.
Some companies are starting to make USB over wireless devices. There are a couple different standards.
But see wireless USB
It is possible to make a device with a push button and a connector for an external contact closure that would act as a shutter release for standards compliant cameras; it might also be possible to support some of the more common proprietary protocols.
Sometimes you need to power a high power device off a low power or even unpowered USB port. Such as a switchable host/slave port on a PDA. In some cases (nokia 770, for example) you need to inject power into both the upstream and downstream ports. One company (cybergear?) makes a battery powered USB hub which might take care of powering the downstream end; it charges from the host end which might be an issue if it tried to charge at the wrong time.
These are not so uncommon. 12V cigarrette lighter and AC adapters with USB ports allow powering or charging devices using an ordinary USB cable. Battery packs are availible. APC makes a USB battery pack, maybe it even functions as a UPS. Yep, sometimes you want a device to stay powered even when power is lost.
One great use for these is to debug USB embedded software on a PC. Another use is to make your PC appear as a hard drive on another system (firewire ports can already do this). Hopefully, USB On-the-go changes will eventually propagate into USB controllers for PCs.
Some suppliers for less common adapters are B&B electronics and blackbox.
usbadapter.com has a lot of different types of USB adapters.