It's nice to come home to a house that is warmly lit, but not very green to leave lights burning all day while you're gone. One day, I picked up a set of outdoor solar lights for $10, and found they also worked well inside.
Just placing the lights in a sunny window lets them charge all day.
Lay them randomly on the windowsill and get cool geometric patterns on the walls at night.
Coil a 14 gauge wire around them and hang them from the valance to get a string of lights in the window.
This could be a nice way to create a starry night-light, without using plugs, in a bath or child's room.
It's a cost-effective way to add interest to a dark room, and save a little sunshine for when you get home.