HTTPS Everywhere is a free Firefox extension–the EFF would like to do it for other browsers but says it’s not possible without source code–that forces the browser to use HTTPS (SSL-encrypted) connections whenever possible. This isn’t foolproof security–HTTPS is vulnerable to man-in-the-middle attacks–but it forces an attacker to do more work in order to snoop on your web traffic.
If you spend a lot of time on public wi-fi networks, this is the bare minimum you should do to protect yourself.
I need to remember to write up an explanation later this week of how SSL is vulnerable to man-in-the-middle attacks. But it’s better than nothing, and there’s nothing wrong with using it as additional protection even when you’re on a safe network.