We use the JY-MCU USB Serial Port Adapter from dx.com. These are extremely cheap but unfortunately the documentation is very scarce (as with almost all DX stuff) so connecting it to an Arduino Mini Pro 3.3V Atmega328 can be confusing.
- Connect USB 3.3V to Arduino VCC. (red)
- Connect USB Ground to Arduino BLK. (black)
- Connect USB TXO to Arduino RXI. (yellow)
- Connect USB RXD to Arduino TXD. (blue)
- Connect USB DTR (Data Terminal Ready, which is basically a RS-232 line) to Arduino GRN. (green)
The VBUS on the JY-MCU Serial Adapter is basically just an extra 5.2V line, handy for Rasberry Pi serial communication.
After wiring everything together select the Arduino Mini Pro 3.3V 328 (make sure it's the correct voltage!) in the dropdown.
And finally select your appropriate bus.
Congrats, you can now start uploading your Blink sketches ;-)