Question: Ok, I have one uart interface (TXD GPIO 14, RXD GPIO 15). I want at least one more uart interface.

Possible solutions:

* Bit banging:
Use two unrelated spare GPIOs. I understand that timing is a problem on a standard linux. Would it be reliable with very low baudrate?
* Switching:
RPI decides when to talk to which device. Using e.g. [CD4066BC](http://www.fairchildsemi.com/ds/CD/CD4066BC.pdf).
* spi to 2 x uart bridge:
Didn't find a suitable part (availability, price, dil package)
* usb to uart:
Expensive

Are there other options? I'm inclined to switching, if it can be done. What would you advise?

Answer: 
A USB UART, such as FTDI, isn't really expensive. All your other options sound like they will cost you more in parts and time than the ~$13 it might cost you, and be unreliable or slow. Just go for the fast and trouble-free option, such as:

<http://www.dfrobot.com/index.php?route=product/product&product_id=147#.UOamLG-sh8E>

Sparkfun sells one too. In fact, maybe you can just pull one out of some old USB device or buy one from a junk store that doesn't know what it does. 

I messed with an SPI to UART adapter for an Arduino project, there was no existing library so I wrote my own. In the end it worked ok, but if I could have just dropped in a $15 part I would have. In fact given the time it cost me I should have just got a mega with 4 serial ports. 

Alternatively if you want lots of serial ports you could look at RS485 serial, which is similar to 232 (though not compatible), which supports multi-drop, ie several interfaces over the one line.
The Raspberry Pi 4 supports up to 4 UART interfaces now that need to be enabled by means of an device tree overlay. You can find how to do this and which Pins are used here for now:

<https://www.raspberrypi.org/forums/viewtopic.php?t=244827>

The RPi foundation is still preparing the documentation for this.