I use Blue Magic Convertible Top Cleaner, it works good. I usually just wash it with soap and water, but i guess after a couple of washes, it brightens the top well, BTW its a 00' gt vert black top. Hope this helps, IMO, its good for the price.
I use RagTopp or something like that. I do have a black top but I have seen it work wonders on a light tan top as well. It's a two step process... you spray on the cleaner... scrub the top, rinse it off, and then put the protectorant on it for a nice shiney look. It's pretty cool to see the top bead water better than the car!
I use a product called "Triple Clean"...its available for like 2 bux...it will clean/remove/protect white vert tops, as well as leather upholestry...I use this stuff all the time and swear by it (a bit can be poured into a rag and used to remove oily hand prints from fenders/hoods without damaging paint jobs)
If you are skeptical, next time you are at a dealership, find the "Make Ready" department and ask one of the guys prepping cars if you can try some...most won't mind (what do you think the use before a NEW CAR is sold?)