I'm sure you've seen night sights that use tritium gas as a light source, which lasts for about 12 years in most cases. There's also fiber optic sights, which use fiber optic inserts to collect and channel ambient light back toward the shooter.
There's also a hybrid system, which uses tritium gas as an "always on" source of light as well as an opening on the sight to allow the fiber optics to collect ambient light. As a result you have fiber optic sights that illuminate under all brightness levels.
I'm quite impressed with these so far. Outside under bright sunlight, these things really glow and it's very easy to pick up your sight picture. I opted for the contrasting colors (green up front, yellow on the rear) to further aid in quickly picking up the front sight. In near and total darkness, they illuminate like any traditional tritium night sight.
If you are thinking about a sight upgrade, consider something like the Truglo TFO. I got mine on sale at Glockmeister and opted to send them my slide for free installation. I don't have the tools to do a proper sight alignment so I figured it was worth sending my slide to them.
Reviewing TRUGLO Tritium / Fiber Optic Sights - Day Shooteta - replaced video with better overview