There are two internet resources that every wine lover ought to know about.
Wine Searcher is a database of retailers from around the world. Their spiders download the latest price lists from every retailer, allowing you to quickly search for wines by any criteria. The basic service is free, and is supported by participating retailers; a $30 "pro" subscription gives you access to the entire database; depending on your budget and shopping habits it can easily pay for itself the first time you use it.
Cellar Tracker is a database of wines. It allows you to keep track of your wine collection, including purchases and consumption, view tasting notes from other users (the database just passed the 1,000,000 tasting notes mark) and lots more besides. Although there are other alternatives, it has become the dominant player. The service is free; a voluntary donation unlocks some additional features. The main criticism of Cellar Tracker is in its user interface; it's not very "Web 2.0" if you'll forgive the horrible buzzword, but this is due to be addressed in an update planned for later in the year. Another thing that is promised is a web API, which really could be interesting.
One of the first significant players to recognise the value of Cellar Tracker was Stephen Tanzer. He allows Cellar Tracker to publish the scores for the wines he reviews; furthermore his subscribers also see the full reviews.
As yet the two other main wine critics (in the US at least) - Wine Spectator and The Wine Advocate - aren't integrated with Cellar Tracker. They each have their reasons; whether I agree with those reasons is irrelevant, but I for one have vowed not to subscribe to their online services until I can access them via Cellar Tracker, so they have lost at least one sale as a result.
Now one of the latest ideas to hit the internet is Vinfolio Marketplace. This is a reverse auction service, whereby users who have wines that they are willing to sell can list them and field offers from interested customers. There have been plenty of sites that allowed users to sell or trade their wines; many such as WineBid or Brentwood act as licenced brokers and charge significant commissions to both buyers and sellers; Wine Commune offers lower commissions but operates in a legal grey area where neither the buyer or seller might have a licence to sell or ship alcohol.
Despite the fact that Vinfolio offers a cellar tracking service, they quickly teamed up with Cellar Tracker to allow users to easily list wines in Cellar Tracker in the Marketplace. They are also negotiating with Wine Searcher on the best way to list wines offered for sale on the site; this is complicated because the wines in the market place are only 'available' - it's up to the buyer to negotiate a satisfactory price. But the point is that they are working together to resolve this.
What I'd like to see is a single portal that combines everything. From one site you could see your cellar contents, reviews from all the critics to which you subscribe, both professional and amateur, current retail prices and recent auction prices, winery details such as that provided by Wine Questers or the ill-fated Appellation America.
As I mentioned earlier, Cellar Tracker is taking about publishing an API. This would allow people to write tools that work with it, in the way that people have written applications that work with Google, Facebook or Twitter. At which point Cellar Tracker may become the definitive online encyclopaedia of wine. It will be interesting to see how long the critics feel they can can ignore that.