The second does about the same as far as i can gather, however: it is NOT breadboard friendly. It has a small pitch and wires have to be soldered on the edges of the board. I have one of those as well, but I use the first one because it's more convenient.
Third same as the second
The fourth is just a pcb with some components, but without a bluetooth chip, hence the low price
The fifth: I can't place it, but I guess also without bluetooth chip, but there is no specification nor is there a picture of the top side
Six: pretty much number one again
Yes there are some extra components on the chip, the JY-MCU has a flashing led to give some sort of indication what it is doing. When connected it stays on. When not connected flashing.
Many IC's have input voltage between XX and YY and will work with everything in between, even without voltage regulator. Atmega328P-PU will work between 1.8 V and 5.5 V
So:
I think what you mean by BASE board is just a board with BT IC.
You can integrate the entire board in a pcb, with either breadboard like sockets (like the arduino has) or solder it on a PCB directly (number two will work, have seen that done)
As you point out the BT module is a 3.3V device. There are different types of base board but most have a 3.3V regulator and/or a 3.3V pin to supply the correct voltage to the BT module. They mostly just use an in-line resistor to drop a 5V TTL serial feed to the RX pin and have no resistor on the TX pin as arduino will detect 3.3V as HIGH. They also have a status LED with limiting resistor and capacitor(s) for the 3.3V regulator. The master version of the back board will ideally have a pin to assert command mode on the BT module and a pin to allow the arduino to know if it's connected of not.
I have attached a datasheet that holds true for the modules I have. Notice the different LED output pin position depending on the module being HC-05 or HC-06. I'm sure KEY is connected to pin 34 and STATE probably connects to both pin 23 and pin(s) 31/32 so the back board will work with both module types. The datasheet also gives connection diagrams for 3.3V & 5V MCU's
xl97:
what are the programmable I/O pins on the BT module used for? (HC-05 only I guess)
I have never used them but I would assume you could connect simple devices like LED's and switches. I have not seen a way you can directly control I/O pins on a receiver from a transmitter so any reading/writing of these I/O would have to happen from the attached MCU.