AirDefense Personal Lite
This freebie promises to help. It monitors the networks to which you connect, and if they're potentially dangerous, issues warnings and offers advice on how to keep yourself safe. It also warns you if you connect to an unencrypted wireless network, or if you've connected to a wireless network that can use encryption, but you haven't enabled it. There's plenty more it warns against as well, such as potential phishing attacks, and even sudden changes in signal strength.
For a free program, this is surprisingly configurable. You can set your security level, you can add safe networks to a list so that the program won't issue warnings when you connect to them, and it lets you blacklist networks, so that you'll never connect to them, even accidentally.
Note that it takes some work to get this software installed. You first need to go to the developer's site, and fill out a form. You'll then get a link via email to download the program. Unzip the file, then run PersonalShellLite.exe to install it. Then when it runs, right-click the AirDefense Personal Lite icon in the system tray and choose Run Now. Then click it again, and choose Show.
Note: The following link takes you to the vendor's site, where you must register to download the software.