Best control breadboarding hardware

Define "best".  It really comes down to what you want to do with it.
For some controls applications, you might want analog outputs.
Other that that, there are many boards available.

If you liked the Arduino Diecimilla, and are already familiar with that IDE,
(http://arduino.cc/en/Main/Software ), then why not stick with the next version,
which has twice the memory (based on ATmega328).
http://store.fungizmos.com/index.php?main_page=product_info&cPathe&products_id#0

For processor eval boards, I like the Luminary Micro (now TI) Stellaris
-- they have one for $50 that you can get through Digi-Key, I found it
very easy to get up and going with.

You may not find much in the way of modular PWM drives -- industrial
practice seems to be to get an all-in-one controller and amplifier (I
could be wrong, I always end up doing full-custom stuff), and a PWM drive
circuit is so easy to whip together that if you're doing something at the
board level you usually just incorporate the amplifier into your
schematic and improve both cost and reliability by avoiding a module.

If you were to find stuff like that it'd be in the back pages of Circuit
Cellar, Elektor, Robot, or Make magazines -- these are all excellent
publications for the serious hobbyist, whose needs are often very similar
to a student doing a project.

--
http://www.wescottdesign.com

I recently bought this for one of our engineers.
http://focus.ti.com/lit/ml/sprt527/sprt527.pdf
 I don't know how good it will be but I wanted to evaluate the TI DSP
for motion control applications.
This may be more serious and not as much fun as you desired.

Another micro controller that seems interesting is the propeller chip
http://www.parallax.com/
You can get most of what you need from those guys.  I haven't bought
one, yet.  This looks like more fun.
I was looking at possibly using a propeller chip instead of a FPGA on
a digital I/O card.

Peter Nachtwey