I have relatively low expectations for privacy in general and privacy on the web in particular. Even so I find I need to lower my standards to cope with the creepiness of Facebook's Beacon feature.
If you have a Facebook account make sure you log out before you do any online shopping.
Facebook has partnered with a bunch of online retailers and is publishing your purchase information to your facebook news feed. That means if you buy a Christmas gift for someone from ebay.com and they look at your facebook page, the surprise is spoiled.
I did some digging to figure out how sites like ebay would be able to figure out who you were on facebook. This post by RadiantCore explains the technical details the details and you can see a demo of how it works here, but the bottom line is when you make a purchase the retail site checks if someone is logged into facebook. If so, it assumes that the logged-in-facebook-user is the person making the purchase and publishes a message on that facebook users news feed that they just purchased a [Insert Inflammatory Gift Item Here].
The ability to do surreptitiously pass information between websites is nothing new, but this does appear to be the first time that a social networking site has partnered with seemingly reputable online retailers to publish your purchasing habits for all to see. Seriously, how does this not violate the standard privacy agreements those sites set up?
Bottom line, to avoid having all your purchases out there for the world to see, stay logged out of Facebook when you're shopping online. As for me I'm staying logged out of Facebook and boycotting online stores that use Facebook Beacon.