As an arduino user do you use other dev boards also, what are the most common "other boards" netduino, propeller, launch pad that you use and why?
I guess anything that is compatible with the arduino software. Most common I would say, Netduino and freeduino.
I personally don't trust the third party boards, even if they say arduino compatible. I research them first and see if they do in fact work with the software, if not then I don't bother with them.
I just started with the Arduino Due as it looks like a very easy way to get started with ARM (Atmel ARM). Also bought a Teensy 3.0 (ARM also - Freescale) but haven't done much with that other than one very simple sketch. Both use Arduino as their development tool.
Other than that, I just use Atmel chips (ATTiny 45/85, ATMega238, ATMega1284, and ATMega2560 with a Schmartboard adapter) directly on pad-per-hole solderboards or breadboards. I haven't actually had the Arduino boards out in months. In the case of the ATMega328, you solder on a 28 pin socket, a momentary switch for reset, a 10K resistor to pull reset up, a crystal and 2 22pf caps, a 10uF despike cap, a few wires, and I solder on a 6-pin IDC header to the RESET/MOSI/MISO/SCK and you are all set!
Other then arduino boards of various colors and flavors. I have a few Picaxe chips (not development boards but does have PC IDE support to program and communicate with them), and I too also recently bought a Teensy 3.0 ARM based board from Paul's kickstarter project. The Teensy is really cute and I bet very powerful for it's size and price, but other then load up the IDE and test that I could upload sketches I haven't identified a project yet that would best utilize it's strong points. So far it's weakest point is me the user, but that applies to my arduino boards also. 
Lefty
I'm using an old STK500 Atmel dev board for development. It works great with the Arduino IDE, has switches, LEDs, an Aux serial (RS232) port. Once I have something working and coded I move it to custom hardware.
Boz:
As an arduino user do you use other dev boards also, what are the most common "other boards" netduino, propeller, launch pad that you use and why?
I've been buying some alternate platforms, but I've just dipped my toe into the programming. I primarily use an Arduino R3.
During the kickstarter campaign I bought the Teensy 3.0 (Arm Cortex M4 based) that I've used occasionally. Right now, I have an issue in that I can't program it on my desktop, and I need to move all of the clutter (read electronics) from the laptop to the desktop on the orders of She Who Must be Obeyed to keep the various craft projects out of the main living space of the house (I do wood working and electronics, my daughter does jewelry making, and my wife does sewing). You can now buy the Teensy 3.0 from pjrc.com. The teensy 3.0 is described here: http://www.kickstarter.com/projects/paulstoffregen/teensy-30-32-bit-arm-cortex-m4-usable-in-arduino-a.
I also bought the Digispark during the kickstarter campaign. This is a much smaller embedded platform, providing just 6 pins and a USB connector. Deliveries should start in a week or two, and I expect I'll probably get it by the New Year. Digispark was described here: http://www.kickstarter.com/projects/digistump/digispark-the-tiny-arduino-enabled-usb-dev-board.
I also bought 2 RadioBlocks for use with communication between microprocessors or the desktop/laptop and the microprocessor. In theory you can program the Arm controller in these boards, but I suspect I will just use them for communication. I expect delivery also by the New Year. The radioblock was described here: http://www.kickstarter.com/projects/545073874/radioblock-simple-radio-for-arduino-or-any-embedde?ref=category.
I bought a TriggerTrap camera shutter release control a year or so ago, and it has an Arduino compatible processor inside. It took TT so longer to come up with the delivery, that I gave up on it and started programming with the Arduino. By the time it came, I had created my own shutter release, and I haven't really used it much. Just recently, I was able to move to 1.0.x of the IDE firmware, and I can reprogram it if I desire.
Also in that time frame, the Rasberry Pi was announced, and I signed up for it. Like TT, by the time I actually got it, I had gotten into Arduino programming, and I haven't done much with it. I keep meaning to go back to it.
Finally, I just upgraded my phone to a Samsung Galaxy S-II, and probably sooner or later I might look into programming it, or at least using some of the apps to use it as a bluetooth controller for the embedded microprocessors.