Pandora

Pandora is the product of the Music Genome Project. From Wikipedia:

The Music Genome Project, created in January 2000, is an effort founded by Will Glaser, Jon Kraft, and Tim Westergren to "capture the essence of music at the fundamental level" using over 400 attributes to describe songs and a complex mathematical algorithm to organize them.

A given song is represented by a vector containing approximately 150 genes. Each gene corresponds to a characteristic of the music, for example, gender of lead vocalist, level of distortion on the electric guitar, type of background vocals, etc. Rock and pop songs have 150 genes, rap songs have 350, and jazz songs have approximately 400. Other genres of music, such as world and classical, have 300-500 genes. The system depends on a sufficient number of genes to render useful results. Each gene is assigned a number between 1 and 5, and fractional values are allowed but are limited to half integers. (The term genome is borrowed from genetics.)

Given the vector of one or more songs, a list of other similar songs is constructed using a distance function.

To create a song's genome, it is analyzed by a musician in a process that takes 20 to 30 minutes per song. Ten percent of songs are analyzed by more than one technician to ensure conformity with the standards, i.e., reliability.

The technology is currently used by Pandora to play music for Internet users based on their preferences. (Due to licensing restrictions, Pandora is available only to users whose location is reported to be in the USA by Pandora's geolocation software).

It's sort of like radio, in the sense that it delivers a stream of music where you haven't picked the specific songs you're listening to and you can't ask it to repeat something you've just heard and there's no direct downloading of the music.

Pandora allows you to create "channels" according to your specifications.  You create a channel by selecting an artist or a song.  It then finds music that is similar based on its genome and delivers hours of listening pleasure.  You can refine it by telling it whether you like the song you're hearing or not.  You can put multiple artists/songs in your specification for a given channel, and you can make as many channels as you want (as far as I can tell).  It allows you pick whole genres.  (For Michael's benefit, I just tested what you get if you just type in "opera", and it lets you select "Opera, classical period".)

Because of licensing issues (or radio regulation?) it won't necessarily play exactly what you asked for, at least not right away, though it may end up in the mix eventually.

Using email addresses, you can "see" and play other people's channels.  (Go ahead and look at mine to see examples.)

You can tag songs you like as you hear them and it keeps track of your tagged list.  There are easy links to access avenues (iTunes, Amazon) to buy the music you're hearing.

The user interface is super simple, which is both a pro and a con.  At first glance you won't be aware of all it can do.

Sure, it's not the sound quality you get from albums or CDs.  And it would be particularly disappointing if you're just playing it on your computer speakers.  (We have a computer hooked up to our "good" receiver/speakers.)  But it will help you find new music, and it's ideal for background music when you're not wholly concentrated on listening and for enjoying lots of music you may not need to own.