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It's surprisingly easy for just about anyone to sit in a coffee shop and snoop on others' activity. That's why an extra layer of security, like a virtual private network (VPN), is critical for browsing on an open Wi-Fi network, especially when visiting password-protected sites. Fortunately, installing a VPN maintained by any of the reputable service providers is simple and cheap.

What is a VPN and how does it work? VPNs create a secure tunnel between the user's computer and an established network or between the computer and a "proxy server" that leads to the outside world. Data (e.g., passwords, location, financial or health records) sent through this tunnel are encrypted and can't be accessed by prying eyes. VPNs are used by business travelers who need a secure connection with the main office network and increasingly by private users who seek the same level of security just because. Setting up a VPN on a computer, tablet, or smartphone is as easy as installing a program and creating a profile.

There are two reasons to use a VPN to reach a proxy server. The first is to protect data traveling on open networks from unauthorized capture. The second is to convince the outside world that your Internet address is that of the proxy server. This "location spoofing" allows access to otherwise restricted content. For example, say someone from the United States has a Netflix account and travels to Europe, where Netflix blocks access. With a VPN service in place, Netflix customers can hide their location by routing the connection request via a server located stateside. Similarly, travelers within (and residents of) countries that block access to social networks like Facebook or Twitter or services like Skype and YouTube can use a VPN service to access these Internet destinations. Using a VPN was a popular means of watching the BBC's 2012 Summer Olympics coverage from the U.S. and is the only way to read The New York Times in Beijing.

There are quite a few free VPN services. The two we like are Hotspot Shield, a well-known and reliable provider that's a tad slower than some but one of the few that doesn't set usage caps, and Security Kiss, which is a breeze to install, is free of ads, and sets a reasonable limit on service. There are also lots of low-cost VPN services. Here our favorites include Private Internet Access (starting at $40 a year), Pure VPN (starting at $50 a year), and Hide My Ass (starting at $60 a year), all of which garner favorable reviews on sites such as Reddit, VPN Mag, and VPN Creative. The latter two VPN services also ranked in the top five in speed tests conducted by a review site called Best VPN for You.

Despite the allure of free VPN, this may be a service worth paying for. Anyone can set up a VPN and offer access -- five friends who live around the country could each set up a server, connect them, and presto: a free VPN service. But as experts at LifeHacker point out, the business model of a free service might give you pause. The provider could be keeping tabs on your activity and you have no control over what it does with the info -- like sell it, perhaps, as a way to keep the operation going. A fee-based VPN service, on the other hand, is far more likely to enjoy steady revenue flows that relieve the pressure to make money off your data. Moreover, VPNs that collect user fees generally provide a higher level of customer service in terms of server locations, uptime, and speed.

Several reputable VPN providers also offer free service as lower-grade options; Hotspot Shield and Security Kiss are examples. Taking this path, however, can mean dealing with ads, limitations on bandwidth, slower speeds, controls over which servers you can connect to, and/or constraints on what you can do via the VPN (i.e., you may not be able to download/upload large files, stream music/video, etc.). That said, this level of service may be worth a trial run to get a feel for how VPNs work and to assess whether free service is sufficient for your needs.

A word of caution: VPNs are also used for nefarious activity. Some individuals hide behind the extra level of anonymity to share copyrighted material via BitTorrent (a peer-to-peer system for illegally downloading music and movies), to commit identity theft, and to engage in other sorts of online crimes. Predictably, such behavior can draw government scrutiny. A recent court ruling found that changing or hiding one's IP address (which VPNs do) in order to access off-limits websites is a violation of federal law. Legitimate VPN providers will turn over information to law enforcement agencies when requested to do so as long as that information is available. Some VPNs, though, advertise the fact that they don't collect any information about users and others periodically wipe their logs clean. If absolute privacy is important to you -- even though your actions are totally above board -- carefully scrutinize a provider's policy on privacy and terms of service before signing on.

While you're at it, check the encryption protocols, particularly if you will be using mobile devices on the VPN or expect to travel to countries that restrict Internet use. Experts say the L2TP protocol is best in these circumstances.

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