It's a very simple design that I discovered more by accident than anything. I was trying to get some form of daylight detector for my "medieval fantasy" build that shall soon be completed and I discovered this.

It is a redstone light with a daylight detector on top of it, with the detector covered on all open sides with some form of solid block. This solid block can be stairs, logs, gold, or the really elusive material known only to treasure as "dirt". Then, when it is fully night and dark enough it will flash... So long as it has a clear line to the sky. It will not flash properly underground.

Additional Notes

This works in 1.5.1, and probably will in 1.6.1. Not entirely sure though.

Works in single player and multi-player. Lots of them will probably not be good for your frame rate though.