The cloud music space is getting crowded. With the Amazon Cloud Player on the scene and Spotify eyeing a U.S. launch, it’s time to get introduced to some of the listening services out there.

Amazon Cloud Player: The newest kid on the block, Amazon’s Cloud Player lets you upload your own music files or buy music from the Amazon MP3 store so that you can access them from anywhere. Amazon will give you 5GB for free to store music, document, video and photo files. Music files bought from Amazon’s MP3 store don’t count toward your limit.

Amazon MP3’s Twitter account gives followers daily deals and advertises free tracks.

Cost: 5GB free, up to 20 GB with an MP3 album purchase

Rdio: Rdio takes music in a social direction by letting users follow each other and build networks. You can follow friends or “influencers,” such as Pitchfork Media or Rdio itself. It’s a neat feature, particularly if you have a friend on Rdio whose opinions you really respect.

Of course, you can also build your own library and playlists from the site’s wide variety of tunes. And you can queue up songs, albums and playlists for a daylong soundtrack.

Cost: $4.99 per month for Web and desktop use, $9.99 per month for unlimited Web, desktop, mobile and supported players

Mog: Mog’s music also plugs into social media, though not quite to the extent of Rdio. You can search for music and compile public or private playlists or use others’ playlists.

A particularly cool Mog feature is the radio slider. Search for an artist’s name so that you can listen to a radio station dedicated to that artist. A slider at the top of the station window lets you add in music from similar artists in whatever ratio you want. So, for example, a channel for the Rolling Stones can also include the occasional track from the Animals or ZZ Top.

Cost: $4.99 per month to listen on the Web or through a streaming device like Roku, $9.99 for unlimited the Web, iPhone, iPod Touch, Android or streaming device.

Grooveshark: Grooveshark is more of a community than a service, since users upload their own music to share. There’s a ton of variety on the site, given the number of users.

The site has run into some trouble, however, over copyright concerns from music labels. Its app has been pulled from both the Android Market and the Apple App Store.

Cost: Free with ads, $6 per month without ads, $9 per month for mobile access

Pandora: One of the most popular online music players, Pandora analyzes your musical tastes and sets up a radio station based on what you like. By telling Pandora a song or artist you’re into, it can build out a station featuring similar music. It does its limitations, however. For one, while Pandora’s super-cool analysis system is good for discovering new music, it can’t get you exactly the song you want when you want it. Pandora’s licensing restrictions keep it from offering music on demand.

Still, the service delivers if you want a mix of comfortable favorites and are looking for new artists.

Cost: Free with ads, $36 per year for the ad-free, desktop and Web service, Pandora One

Got a favorite online music service we didn’t mention here? Share it in the comments.

Related stories:

Spotify cuts free listening time in half

Amazon Cloud Player puts your music on the Web

Report: Google testing Google Music