I just purchased the GreatPoint 4 LED rechargable light. It is not heavy at all, and clips onto the standard PRS-500 case just fine. The light is fairly even on the Sony reader, and illuminates the entire page when clipped to the top. It has two brightness settings, and I find that the lower setting is easier on the eyes (less reflective glare).
My only complaint is that, on the charge cradle (which is a bit heavy - probably weighted so the unit won't fall over when charging), the charging light is always on - even if the unit isn't in the cradle. The unit was a tad pricy - I paid $35 for it at Barnes and Noble, but I had a gift card so it wasn't nearly as bad.
If anyone is interested, I can take a few pictures of the unti in use.
Mike