I'm having a lot of fun playing with my new Arduino, and have created several projects using the few LEDs that I have.
I mentioned my "fake sunrise" project a few posts back, and that's one of the things I've started and am reasonably satisfied with. A few things to work on – making it easier to set the time and delay (right now it requires reprogramming the chip, I want buttons and a display instead), getting a more powerful power supply (the LEDs are brighter when it's being powered off of my laptops USB port), and getting a better supply of resistors (I'm using 330 ohm resistors where I should be using 68 ohm and 153 ohm resistors, not a big deal, the LEDs just don't light up as bright as they could.) It's more of a "fake moonrise" right now, but with a little bit of work I think I can double or triple the light output, which would be great.
Using the same setup, I've also written a program that randomly fades each of the red, green, and blue LEDs off and on. This looks pretty cool in a dark room, and will also benefit from a different power supply and a more appropriate set of resistors.