Though most of us have Web sites we revisit often, a lot of the Web is what we’d call works in progress. There are, however, some Web sites so wonderful that they seem to call us to a new and better world.

Pandora is such a site. I remember my friend Jim Cherniss telling me about the site a few years back and thinking it was pretty cool, but I believe it’s really arrived now, really reached critical mass.

You see, Pandora is a work in progress because it can never be finished.

Pandora is more than an Internet radio station. It’s a radio station that plays only what you want to hear.

When I revisited it the other day, it asked me what kind of artist or song I wanted to hear. I typed in Diana Krall, the name of the Canadian jazz pianist/songstress. I immediately got Diana singing a beautiful rendition of “Willow Weep for Me.” But here’s where it gets interesting.

Pandora followed that song with an Ella Fitzgerald track, clearly the kind of tune that Diana Krall might have been inspired by. Next, Pandora played a haunting Billie Holiday tune. And then a Blossom Dearie track. Then it followed up with Julie London, and then Nina Simone. Next came several I’d never heard of but very much in the same groove.

What I was listening to was the perfect radio station for the Diana Krall fan. Each track possessed most of whatever I liked about Diana: that phrasing, that coolness, that sophistication.

How does Pandora do it? It does it through its Music Genome Project.

The Music Genome Project is an undertaking in which about 50 musician-analysts, all of them highly trained and experienced, rate songs based on more than 400 distinct musical characteristics, such as melody, harmony, instrumentation, rhythm, vocals and lyrics.

The Music Genome Project claims the typical analyst in its employ has a four-year degree in music theory, composition or performance, although the kind of skill the playlist masters of Pandora demonstrate reveals a tremendous musical intuition, as well.

The goal of the project is to create a taxonomy of musical information that will be used to analyze and catalog no less than several centuries of music, all for the ostensible purpose of presenting to us, the listeners, just whatever we want to hear.

The project is delivering in spades. Last Friday evening I entered “Beethoven.” Pandora played the slow movement from Beethoven’s Piano Concerto No. 1 in C major, a piece I happened to know and like.

So I decided to tell Pandora to play more like that one, which Pandora lets you do through its “Guide Me” feature. I got three slow piano concerto movements in a row, including a Schumann and a Mozart. Perfect.

Next time you want just the music you want to hear, go to Pandora.com. It’ll make you want to invest in some really good speakers for your computer. My Logitech Dolby Digital 5.1 set was really put through its paces.

Phone users, especially those using AT&T or Sprint, can also listen via their phones anywhere they go. Pandora says you can try it out for free.

Now I finally get the name. Once you try Pandora, it might be instant addiction. It’s like opening Pandora’s Box.