Just a quick note on some of the driver issues and finding cheap ways to do it. This is specifically to address (or just add to) kcress's post from about 20 pages back... sorry it's a bit of a necro.
There is an adjustable regulator which can be had for about $1.00 called LM317. This has 3 pins IN, OUT, and ADJ (adjust) and can run over 30 volts at 1.5 amps. This is perfect for driving those 6-LED strings at 24V. You simply wire a 1.25 OHM resistor to the OUT line, and then the LED after this. The ADJ line is tied between the resistor (just after it) and the first LED. This gives you a 1amp CURRENT controlled regulator. Double to resistance, and you halve the current... easy math. You can then look at potentiometers instead of resistors for fine control.
The advantage here is that you should really have a current controlled driver, and the I-V relationship in LEDs is highly nonlinear. A small change in voltage makes a very large change in current which can quickly run away and kill the LED. The second thing.. a blown LED (short) will cause an increased current flow in a voltage controlled source, and could blow the rest of the string. Better than a fuse, this source will still drive only 1A, even if 2 blow. A simple resistor will start running the remaining LEDs too hot in this case, and may take them out too. The driver basically acts a resistor that changes is resistance to makes sure that the resistor always has a 1.25V drop across it which is 1A for 1.25 Ohms, and therefore the LED string will always run a 1A... This also lets you reduce the power dissipation of the resistor (1.25V x 1A = 1.25W). The LM317 also has internal current limiting and temperature sensing, so it will shut itself off before it fails in case anything goes wrong. This makes the circuit more fail-safe and even cheaper.
Another idea for those trying to make dimming circuits is to use PWM. This essentially turns power on and off rapidly, giving the effect of dimming. This can be done very easily using a micro-controller and a power MOSFET. The MOSFET (520N for instance) can typically handle 10A, more than enough and is again $1-2... but what if you don't have a microcontroller? There's another simple and cheap circuit you can use.
The idea is a little more complex, but still easy to do at home. The well-known 555 timer can be used to generate a "sawtooth" wave, basically a triangle wave instead of square or sine waves. That output is fed to one side of a comparator, which simply a switch than turns its output on and off based on which of 2 signals is larger. The other side of this comparator gets hooked to the center tap of say a 10k potentiometer which is wired to the supply and ground. The output of the comparator is the finally hooked to the gate of the MOSFET. Lets say we set the potentiometer half-way up. The triangle wave will then spend half its time below that voltage and half above it. The means that the comparator, and therefore the MOSFET will spend half its time on and half its time off, which leads to 50% output on the LEDs. This gives two big advantages... The first is that for those using white LEDs or various different colors of LEDs, the color drifts ALOT less when using PWM dimming... and yes color does depend on the output due to a different temperature of the LED. The second is that the same triangle wave can be fed to any number of comparators, and you can get up to 4 or even 8 comparators on 1 IC, which means very little additional investment to control multiple colors and/or channels.
I've attached a sample circuit diagram for the sawtooth oscillator for you, but triangle waves work equally well. The frequency is almost unimportant for this application, something between 100Hz and 10kHz will be fine, so component selection is simple. For the comparator an LM339 is both cheap and effective.
For a total price of around $20 you can build a 4-channel control system and driver sets for almost as many LEDs as you could possibly want to run (plus the price of the 24V power supply) and have flexibility and fail-safety. A little more complicated the kcress's design but still easy, even cheaper and a makes me feel a bit more secure. You may not want to tackle PWM, but I high recommend you try a real current-limited supply instead of a simple resistor, its only 1 extra part.
