The most popular free VPN service is the cross-platform Cyberghost VPN as of 2015. On BestVPN.com, Cyberghost was named the best free VPN for Windows, Mac, Linux and mobile because of its high speeds and unlimited use. Other highly rated free VPNs are SurfEasy, TunnelBear and Hotspot Shield. Although these services don't have speed caps, they all have data caps and are ad-supported.
Cyberghost VPN has no data cap, but it does have a speed cap and is partially ad-supported. Cyberghost is known for having high security because it doesn't keep logs and even deletes payment details after they're processed. Cyberghost is available for unlimited free use on Windows, Mac, Linux, Android and iOs.
The second-best VPN service according to BestVPN is SurfEasy. SurfEasy is a Canadian company that has high speeds with no limits as well as great privacy and security features. SurfEasy's free VPN has high speeds but has a monthly data limit that can be increased by performing small tasks. All paid plans support five simultaneous connections, and SurfEasy's client is known for being easy to use. SurfEasy doesn't support P2P clients to ensure its users' security.
Other free VPN services similar to SurfEasy are TunnelBear and Hotspot Shield. TunnelBear is extremely similar to SurfEasy, with high speeds, data caps and no P2P support. Hotspot Shield is a popular service for its easy to use software and lack of data cap; however, the free version only connects to U.S. servers and is supported with ads.