I had quite a good success with wire wool (stahlwatte). I bought it, I think, at Migros and used it to clean the glass hob. Didn't leave any scratch marks and removed or at least smoothened a scratch.
Zuzi: are you sure it's a scratch? Sometimes an Aluminium pan will leave a smear of Al on the glass. These will usually come out with a felt buffing pad on a cordless drill, glas hob cleaning cream, and some patience (ok, sometimes lots of patience).
You can usually feel the difference between a smear and a scratch with your thumbnail. A scratch will definitely catch, a smear just adds some resistance.
Zuzi I use this product, its from the "Migros" . If you clean your hob a few times with this, your marks should be less visable. Its also good for cleaning away chalk stains and ovens. A sponge is included.
I use the same sponge, have been for years without any issues. But then again, the glasstop producers recommend something like that scraper thingy linked to by roegner.
With that said, avoid overly aggressive cleaning fluids (say, oven cleaners), use something like ordinary dishwasher instead. And if you keep using the sponge make sure there's no sand, e.g. by wiping the glass with a wet cloth before using the sponge with pressure.
Oh dear, I used that on a car once, on metallic paint . When I was very, very young. I didn't even get the chance to talk my way out of it, the guy at the garage started our conversation with "you used the green sponge on the car ..." LOL.
If you use that sponge on your cooking hob, make sure to also use a lot of water (a pro told me that once and she was right) and don't scrub too hard. Well, that's for next time anyway