Redstone powered. It automatically comes on at dusk and off at sunrise. Very compact and simple in it's design, and easy to build when you have to make hundreds of them. It is attractive without being overly obnoxious and clunky. I recommend placing them every 40 blocks for a good appearance and even light distribution, with the posts being 7 blocks above the roadbed.
This is entirely my own design I invented around autumn 2013 or so and have been sitting on this ever since. Wasn't necessarily a big secret, just didn't feel like anyone would care. Now, I figured I should share it with you all as it really is very helpful for road and city building!