Risks of Using Unsecured Wi-Fi Networks
If you log into a website or use an application that sends data in clear text over the network, that information can be easily captured by anyone motivated to steal another person's information. Your email login information, for example, if not transferred securely, would allow a hacker to access your email, and any confidential or personal information in your account, whenever they want -- without you knowing. Similarly, any IM or non-encrypted website traffic can be captured by a hacker.
Also, if you don't have a firewall (or it's not configured correctly) and you forget to turn off file sharing on your laptop, a hacker can access your hard drive over the network, accessing confidential or sensitive data or launching spam and virus attacks pretty easily.
How easy is it to hack a wireless network?
For about $50 you can get the tools needed to learn all about a wireless network, capture (sniff) the data transmitted on it, crack the WEP security key, and decrypt and view data on network devices. Lucid Link, a wireless network security software developer, has a Flash demonstration of what wireless hackers can do.
Is it legal to use someone else's open wireless network?
In addition to the security issues above, if you hop onto a wireless network someone else is maintaining and paying for, there may be legal issues involved. In the past, several cases of unauthorized access to wi-fi computer networks have resulted in fines or felony charges, as About's Guide to Wireless/Networking has pointed out. If you use a public wi-fi hotspot set up specifically for guests to use (e.g., your local Starbucks), you should be fine, but keep in mind you'll still need to pay attention to wi-fi hotspot security issues, since wi-fi hotspots are typically open -- i.e., unsecured -- wireless networks.