eLight standard for BMW 5 Series E39
Had this installed on my 2001 530i for about 7 months but took it out when when the car was written off.
It allows you to individually control how your exterior lights turn on and off when you lock and unlock the car (you can do some pretty elaborate stuff, do a search). I just wanted my angel eyes to stay on for a few seconds after I locked the car (looked cool).
It also allows you to to use your fog lights as daytime running lights. I did this as well and really liked the way it looked (with the standard daytime running lights disabled).
Asking for $150.