I began all this with Arduino modules. There are a variety of Arduino "shields", which are daughter boards compatible with the pin layout on the Arduino modules themselves. I have never worked with any of these, but it is something worth remembering as the prices are right and software has already been written.
I began all this with a hand-me-down Arduino NG rev C board (based on the ATmega168 chip).
I soon ordered my own Arduino Duemilanove, which at the time I ordered it had an ATmega328 chip (older units used the ATmega168 chip).
This is a lot like the Duemilanove, it still has an ATmega328. The board is the same size, has the same signals (or so it seems) on the edges. The difference is that it uses surface mount chips (unlike the socketed DIP packed ATmega328 chip on the Duemilanove). This change was apparently a big deal for some people who were in the habit of blowing out their controller and popping a new chip in the socket.
A significant change is that the Uno uses the ATmega16u2 chip for the USB to serial interface (rather than the usual FTDI chip). The benefit of the 16u2 chip over the FTDI, is that it is relatively easy to reprogram it. This allows you to change the USB vendor and device codes, allowing your project to configure itself as any kind of USB device, not just the USB to serial device you were stuck as when using the FTDI chip.
My big beef with the Pro-micro is that the micro-USB connector is very fragile. Mine ripped off the board the second day I was working with it, under minimal provocation. Many others have had the same experience, you have been warned.
At one time, I was itching to get my hands on a Boarduino from ADA Fruit, but the 32u4 breakout has cured me of that. The appeal of the Boarduino is that it is a tiny module that can be plugged directly into a standard breadboard. However it uses a DIP package ATmega328 chip, and has no onboard USB (so you need a FTDI cable or module or some such to program it - something like the FTDI friend from ADA Fruit).
Not as exciting, but more amusing and unique is the StealthDuino.
Tom's Computer Info / firstname.lastname@example.org