Use as a weaponEdit
The four varieties in increasing order of damage are: green, blue, red and double red. They are a high-damage weapon, but have a -3 to-hit penalty (-4 for double reds). They turn off automatically when they are unwielded, although if you are fast enough, you can dual-wield them, or even stick one in you bag of holding! (Note that this currently does not damage your containers in any way...)
Use as a toolEdit
Lightsabers can be used to burn Elbereth and to burn through doors, making them valuable, even when not used for combat. Note however, that they can run out of power! Charging them works like an oil lamp: 750 turns from uncursed, 1500 from blessed to a maximum of 1500 turns.