After having a visiting family member slip on our dark stairs in the middle of the night, we immediately decided to install some sort of lighting. No one was seriously hurt, but it was a wake-up call. I decided to put louvered step lights at the bottom and mid-way up the stairs.
These units cost about $25 each, and are fairly easy to install. Running the wiring under the stairs is also a lot easier than breaking out sheetrock.
Here is a picture of it installed in the wall and turned on.
The picture below shows it without the cover on.
There are a number of options for these type of lights, including smaller ones. I decided to go with the larger 5" x 8" version that uses a standard 120V bulb. With a 7 watt CFL, they don't take much power - and they put out quite a bit of light (40W incandescent equivalent).
Instead of putting the lights on a manual switch, I found a sun-down to sun-up electronic timer to control them. This one is made by Intermatic and cost about $30 at Home Depot. There is a slightly cheaper one, but is specifically not for use with CFL bulbs.
After I put these in, we liked them so much, I extended the circuit. It now includes lighting from the kid's bedrooms downstairs, up the stairs, and into the kitchen. This creates a "light path" throughout the house, that everyone has appreciated.