This board connects directly to the FTDI Basic Breakout board and supports auto-reset. The Pro Mini also works with the FTDI cable but the FTDI cable does not bring out the DTR pin so the auto-reset feature will not work.
Features:
• ATmega328 running at 16MHz with external resonator (0.5% tolerance)
• USB connection off board
• Dimensions: 0.7 x1.3" (18 x 33mm)
• Supports auto-reset
• 5V regulator
• Max 150mA output
• Over current protected
• Weighs less than 2 grams!
• Reverse polarity protected
• DC input 5V up to 12V
• On board Power and Status LEDs
• Analog Pins: 8
• Digital I/Os: 14 Spec sheet available in PDF format.
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ARD-18 "20 MHz" Pro Mini 
Despite saying that this is a 5-V 20-MHz board, it seems to work fine with the standard Arduino ProMini 5-V 16-MHz board selection in the IDE. I checked it with a DHT22 temperature/humidity sensor, which is sensitive to board timing issues, and it works as expected. The pin descriptions are printed only on the back side of the board, which makes it more difficult to work with if you add headers and plug it into a breadboard. (Take a closeup photo of the back of the board before you add headers.) You wouldn't want to mount it upside down on a breadboard, with the pin descriptions showing on top, because that would eliminate access to the reset button. It does support I2C devices through pins A4 (SDA) and A5 (SCL). There are cheaper Pro Mini clones available all over the web, but this board is OK and the $5 price is not bad. As noted, like the standard Pro Mini, you will need an FTDI cable or basic breakout board to program it.
Pro-mini bootloader 
I recently bought 2 of these which were a bit different than other ones I had purchased. I had to burn the bootloader into these Pro-mini's to get them to program. They would then load a program, but only once. I have to load the bootloader each time to upload to these 2. All the other Pro-mini's obviously are different. Any clues? Thanks. Excellent little computer - I will get this solved!
Works great! 16MHz not 20MHz
As another reviewer noted, works well with Pro-Mini 5V/16MHz selected in Arduino. That is because it is a 16MHz board, not a 20MHz board. I verified the frequency with a scope. It is using a 16MHz oscillator.
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