StumbleUpon is a popular Internet community that allows its users to discover and rate Web pages, photos, and videos sharing. Let’s see what are the proxies that this community promotes:

• Your Proxy! (IP: 67.159.41.89, Location: Woodstock, IL United States) 
• SeeTHROUGH (IP: 67.159.41.89, Location: Woodstock, IL United States) 
• PuzzleProxy (IP: 67.159.41.89, Location: Woodstock, IL United States) 
• Corpask.com (IP: 67.159.41.89, Location: Woodstock, IL United States) 
• Orkut.info (IP: 67.159.41.89, Location: Woodstock, IL United States) 
• 99open.info (IP: 208.53.147.16, Location: Bronx, NY United States) 
• AnotherFreeProxy (IP: 66.90.104.89, Location: Woodstock, IL United States) 
• 99browse.info (IP: 208.53.147.16, Location: Bronx, NY United States) 
• Peanut (IP: 67.159.41.89, Location: Woodstock, IL United States) 

As you can see there are only 2 proxy networks that I could find on a quick search. Probably the owners are power users on StumbleUpon.

Anyway, enjoy the free proxies listed above!

Happy Surfing!