Amongst the other temptations and meatballs found in the typical IKEA store is a small lighting section, and within there is a combination of LED light strips with a handheld controller unit that goes by the name of "Dioder". The whole kit is around $50 and is quite good value - and also easily hackable. Why bother with a simple manual control when you can drive it from a PC or other device that can have a serial interface with an Arduino? This is exactly what Fritzing member "MARTISAK" has done - and by publishing the schematic, PCB file, Arduino sketch and python code you can also recreate your own version. The controller relies on serial text from the host device to control the LEDs, and is thus quite customisable.
If you're looking to make more creative designs with LEDs, consider the Freetronics RGBLED: full colour module. It includes a bright RGB LED on the top of the board and a WS2801 constant-current, addressable, multi-channel LED driver on the back. This smart module can be daisy-chained, so you can connect a number of these together in a string and drive each of the module colours individually from your microcontroller. For more information and to order, visit the product page.