Alas as my father has often told me - nothing is really ever free. This is entirely correct I mean why would anyone spend lots of money providing a free proxy service for me ? There are a few (and I mean very few) legitimate free proxy servers on the internet and they are so painfully slow to use that you'd have to be ultra paranoid to use them.
So why set up a free Proxy Service?
The payoff for a lot of these proxy servers is either stealing the data and identities of their users or bombarding them with adverts or malware, neither really appeals to me. It's a fairly low risk criminal attack as well, because many of these servers are merely hacked and an open proxy installed.
We all know you get what you pay for, but in this particular situation paying nothing is really a very expensive option. If your concerned about internet privacy then using a free proxy service is worse than doing nothing, if you don't mind a huge speed impact on your browsing use JAP or TOR, fairly secure but unfortunately pretty slow (although it is only to be expected)