I recommend two tail lights that work really well.

The Planet Bike Superflash (PBSF), has these options going for it.

1. It is really bright for its being a small, battery-operated tail light.
2. It can be easily clipped or mounted anywhere on the rider, the bicycle or its bags/trunks/paniers.
3. It is lightweight.
4, It works for a long time on one set of batteries.
5. It is cheaper to buy but its quality is good.

The CatEye TL-1100 also has these going for it.

1. It, is large, bright and is more obvious.
2. It can be mounted to a rack, seatstay, or clipped to a bag or panier.
3. It is heavier.
4. It has more patterns and features than the Superflash.

You can't go wrong with either one. I bought three Superflashes and mounted two of them to the chainstays of my bicycle. I have the third one clipped to my helmet.