There are a lot of cool Arduino projects floating around Eugene Maker Space. You can do a lot in a hurry with an Arduino, and programming is so easy it almost feels like cheating! Here are a couple simple projects I've done.
I had less than a day to design and build this prototype from scratch and fly to California to demo it. Normally I would design and build up a custom board, program a micro in assembly, make a 3D model of the mechanical design, and machine a custom case. None of that was an option, so I rushed out and picked up an Arduino from Radio Shack and decent looking junction box from Home Depot. Even having never used an Arduino before, within a few hours I had a completed product. A few LED's and an ink jet printed face plate and the result was remarkably professional. Unfortunately due to the commercial nature of this, I can't show too much of the final result.
This application is a perfect fit for an Arduino. There are 4 LED's that flash and fade to give user feedback. There is a button input, as well as a light sensor. An op amp and trim pot are used to amplify the output of the photocell. There is also a relay to switch a line voltage device. This is soldered together on a proto shield sitting on top of an Arduino Uno and powered by a 9 volt battery.
Here is another example project. This is an Arduino Mega 2560, which has a lot more I/O and memory than the Uno. Connected to it is an 16x2 LCD display from Adafruit. What is notable about this display is it has an RGB backlight, meaning you can change the color to anything you want, including fading from one color to another. This is also a 'negative' display, so the characters light up instead of the background.