When you piggyback on someone else's wireless router, you're joing their local network. It may have just one personal computer on it, or you might accidently connect into a corporate network with hundreds of other computers visible. While you're connected to that network, go look at Network Neighborhood (or whatever, changes w/each version of Windows) and see if other computers are visible. If they are, you might find that you can look at their hard drives, if they're not properly secured.

They could do that to you, of course. Every vulnerability revealed this way is a vulnerability you share.

There's a thing called a "packet sniffer" that listens to (and can record) all traffic on a local network. Network cards have to listen to all traffic anyway, to recognize your traffic; packet sniffing just turns off the filter and makes the network card "promiscuous". Somebody on that other network could be running one (there are smple software-only sniffers that you can download from the Internet), and catching the addresses of your connection attempts, not to mention all the sensitive data that could come back. Similarly, they can watch your email traffic. They couldn't interecept your keystrokes while you're typing that message, but of course they could read the message as you send it.

You could disrupt their network because you're sharing a single Internet connection. Every time you downlaod a DVD or music CD (or to be realistic, a big pile o'FITS), your neighbors will see their connection speed slow to a crawl. They might wonder why, and look at their router's administration screens to find an unknown computer listed as currently connected. If you happened to have named your computer "Robert" or "LaserBreath", they might even have a good idea who to ask about it.

As you can probably tell, I don't consider it worthwhile to borrow wireless service.

From the other direction, you can secure your own wireless network pretty well following the suggestions posted here. Use WEP (Wireless Encryption Protocol), with the largest key-size the system supports. Change defaults like network name and administrative password to something unique. Set your firewall to present a teflon-coated face to the Internet--no ports open--then open up only the specific ports you find you need to do specific things. It's possible to set your firewall so tightly that the ShieldsUp test at Gibson Research (grc.com) mentioned by SDG tells you that your IP address is essentially invisible on the Internet (full "stealthed" in firewall terms).

Some wireless routers let you choose between various "membership" levels: Any computer can join the net (the "Net cafe" model), computers can join if they know the network name, ...if they know the net name and encryption key, and--finally and most secure--nobody can join unilaterally no matter what, you have to specify exactly which computers are members of your network. Use the last and strongest model--you know who is and isn't allowed on your home network.