If you use wireless in public location (like Starbucks), it’s possible that someone else can use a Firefox extension called Firesheep to “sidejack” your Amazon, Facebook, Twitter or Google account (among others). This allows them to do things in your name on those accounts, and perhaps even steal your password (under rare circumstances).
Though the vulnerabilities exploited by Firesheep have been around for a while, once an easy-to-use tool is released, it’s pretty common for jackasses to make heavy use of those tools for malicious purposes. The simplest cure is this tool from the good people at EFF. Here’s a more lengthy explanation of how Firesheep works and what it can do.