How did they do this??????

[Druffus]

If you go to http://www.bluestraveler.com Chech out the menu at the top of thepage. It seems that they have a background with a layer over it and whenever the cursor goes somewhere on it, then it has a trail showing the underneath background. I have a few ideas on how it could be done, but I would really like to know exactly how. I think it is a great effect.

[airheaded]

I would say a animation on mouse over. But i could be wrong.

[Perlaki]

It is an animation on rollover. There's just loads of square buttons along the top, and in the rollover state, there's a movie clip.
As you say, there are lots of nice effects you can create doing this.