Open networks are still a gray area in both law and ethics, as no laws have been enacted that control how they should and should not be used. There are any number of opinions as to their legality and ethicality, so you'll have to decide for yourself.
Whether it's legal to open your network to the public, or even to selected friends, depends entirely upon your ISP user agreement. Unfortunately, not every ISP has been clear about sharing access (especially with older agreements), but you can almost always find something on their web site describing usage policy. While some ISPs explicitly allow you to share the wealth, others explicitly ban it. If you're with one of the latter, you'll need to weigh the benefits of opening your network against the chances of being caught.
In any event, if you do choose to make your network open to others, consider giving your network a name that makes your intentions clear. For example, you could name your network something like "Public HomeNet," or "Use My Bandwidth, Please." You can also set up a Mac with a web server on your local network that is Rendezvous-enabled, so that visitors using Safari can easily find it. On that local site, you can include information about your wireless network, and a bit about you. See Chapter 8 for more information about using web sharing as a billboard for your WLAN.
You're on the road, need access to the Internet, and find an open network where someone hasn't enabled encryption! What do you do?
This is an area on the edges of the legal frontier, and many people disagree about what's proper when it comes to using open (but not explicitly shared) networks. The arguments for and against piggybacking on someone's AP usually come down to analogies: is borrowing bandwidth like listening along with your neighbor while they have their radio on, or is it more like breaking into their apartment while they're out and making copies of their CDs?
Two examples from the front lines:
The World Wide War Drive group (http://www.worldwidewardrive.org/) organizes an irregular national search for unsecured APs, with the goal of teaching owners how to secure their wireless networks. Their page on ethics specifically requests that participants not connect to any of the open hotspots that they find.
On the other side, the legality FAQ at Warchalking.org argues that using open networks is both legal and moral (http://www.warchalking.org/story/2002/9/22/223831/236). After all, if the owner doesn't want their network to be used by the public, all they have to do is turn on WEP to make their intent clear.
The jury is still out on this one, and we aren't qualified to give legal advice?you'll have to decide for yourself where you stand.