Simulate any USB device using a laptop/PC?

I was wondering if anyone knows of a sort of 'digital toolbox' piece of software, that allows you to simulate other USB devices. 

I was thinking you would plug in a USB to USB cable from say a laptop to a desktop, and run the software on the laptop to simulate any number of USB devices (external HDD, WiFi/Bluetooth dongle, Keyboard, etc etc).

Would this be possible / does it exist?

 

There are two types of USB devices, Hosts (like you computer) and slaves (mice, keyboards, USB drives etc). Although there are some devices that can act as either (like some mobile phone) you cannot plug one Host into another Host and have it simulate any slave.

Speedfox's comments about host vs client are correct. But more than that, your cable's configuration is part of it too! So be sure you're not dealing with old USB specs or you may have to make a "null" USB cable. (And be very careful you don't mix up the +5v and ground too if you happen to decide to make one. That would be very bad.)

Now, if all you want to do is get files from one computer to another I can think of at least a half dozen better ways than using a USB port...