Most of the proxies are full of ads to increase the owners revenue. Honest it is not bad. Proxy owners gain money from ads and visitors get services from the proxies.

But now a lot of ads expecially pop-ups contain a lot of viruses, trojans and malwares to harm visitors computer and steal their identity, credit card information or banking details.

At http://sites.google.com/site/adfreeproxies/ they choose some really nice ad-free proxies from Google and list them for people to surf websites far away viruses, trojans and malwares.