I use CellarTracker to keep track of wines that I like and wines that I have in my collection. With it, you can access access community tasting notes and ratings, or leave some of your own.

There's also an iOS app, Cor.kz, that lets you access the site on the go. It's perfect for scanning UPC codes on bottles of wine in the store before you buy it to make sure you're buying a decent bottle of wine.