I had a bit of a debate with myself about whether this should go in MOCs or sets, but decided the changes constitute a MOC, even if it's built on a set.
I've lit up the engine of my UCS Millennium Falcon:
Lighting the UCS Millennium Falcon by James Shields, on Flickr
It's done using a €2 20 LED set from a Euro store. I had to replace the 2x3 grey plates the trans-blue tile attach to with transparent parts (and as 2x3 clear plates cost a fortune, I built them out of 1x1s and 1x2s). After that, there was a bit of work to remove some parts that blocked the lights, and some internal parts to hold the wires in place. The battery box sits neatly under the top gun emplacement, where it's easily accessible to turn on/off or change batteries.
For a fairly easy mod, I think the really adds something to the Falcon.
James