Serial communication between multiple devices (or Arduinos)
Almost any serial communication system can be "bent" to make a multi-master system - some easier than others.  The biggest problem you have is that of detecting and handling collisions. 

However, if you can get away with a master->slave(s) system whereby one node on your bus makes requests to the slave nodes to get them to do something and respond with information then the whole system is far far simpler.

That is the normal way that I2C and SPI work, although I2C does have multi-master in its specification (though the Arduino API doesn't handle it AFAIK).

The industry standard way of making a UART able to communicate with multiple devices is through converting it to a standard bus system, such as EIA-422 or EIA-485 (AKA RS-422/RS-485). A half-duplex multi-drop EIA-485 is probably the simplest to arrange, and there are breakout boards for the Arduino that will convert UART to EIA-485. 

It is possible (if you are clever) to make UARTs directly work as a multi-drop bus, but it requires a deeper understanding of UART control registers and electrical characteristics to emulate either an open-collector or tri-state TX pin on the slaves.

Of course, for all devices to operate on the same bus they must all conform to the specified operation of the bus. For instance, for EIA-485 all the slaves must have the ability to switch the transceiver into TX mode when needed, and react only to requests from the master and never send anything unsolicited. Just hooking random devices up to a bus with a transceiver is never going to work. 

As far as USB goes - that is a single master (the PC) and multiple slaves (your keyboard, mouse, printer, scanner, webcam, Arduino, etc). Hubs are just a way of wiring them all together and routing the signals to the different peripherals at the right speeds.