Times are pretty good for people who want to listen to streaming music. Between the services MOG, Pandora and Spotify, there are enough features and a variety of tunes to satisfy almost everyone. But which is the best, and for what? We take a look.
Pandora Radio
Pandora is a US-only automated radio and recommendation service. Their service is built around the Music Genome Project, in which any given song is represented by a list of 400 attributes ("genes") of musical characteristics. When a song is presented to a Pandora user, they can indicate whether they like it with a thumbs up or down, and the service takes that into account for future recommendations.
Pandora has Apple and Android app versions, and has two subscription options: free with advertisements and a 40-hour cap, and paid.
Pros: We've found the recommendations system works really well, and have even discovered some new favorite bands that way. The service plays you types of songs you'll like in any genre you want, so it's a good way to explore. The preset radio stations are also worth a look.
Cons: Can't control which songs you listen to, which is a bummer if you really do just want to hear one artist or album and not similar types of music. If you listen to the service every day for a while, songs start to repeat, or recommendations get a little wonky (along the lines of playing Disney radio stars near the end of a hip hop mix).
Spotify
Spotify is a Europe-only music streaming service with a wide variety of music from major and independent labels. Currently they offer about 13 million tracks, with about 10,000 added each day.
Spotify has free and paid subscription options, and plays through a dedicated player available on every OS as well as Android and Apple phones (with a premium account).
Pros: We have been massively impressed by the sheer amount of music available on this service; Spotify has just about every track you'd ever want to listen to. They also have a really great playlist-making and -sharing setup, which is as easy as copy and pasting a link. Playlists can also be edited by groups of people.
Cons: Obviously being Europe-only (and only Sweden, Spain, Norway, Finland, France, the Netherlands and the United Kingdom at that) is problematic. This worked for us while we were traveling back and forth between continents a lot, because logging in from a European IP lasts for about two weeks before it checks where you are again. Supposedly they're going to release in the US some time in 2011.
There are some other problems with content restriction from record company licensing, and they've recently announced free users won't be able to listen to as many tracks as before, or as many times.
MOG
MOG is a paid subscription and social music service. MOG has no regional restriction as far as we're aware, and because it's paid there are no ads. The music catalogue is 10 million songs so far, and includes full albums. Users can either queue songs and albums, or listen to more from the same or similar artists.
Subscriptions are $5/month for web and Roku channel access, and $10/month for listening through those as well as apps. Users can download songs to the app for offline listening. New users get a 14 day free trial.
Pros: MOG seems to be the best of the Pandora and Spotify worlds; listen to whomever you want and when, and build playlists like Spotify offers, or have slightly-less-accurate-than-Pandora radio-listening options.
Cons: If you're willing to fork out the cash, it's a pretty good deal, but paid still has a hard time standing up to free. Similar artists can be a bit random at times.
Do you use these music services, or any other? Which do you think is the best? Let us know!

Ercol Bar Stool
rdio.com is my pick!
Another vote for rdio.com I'm surprised that you didn't mention it when it's by far the best service available in the US.
I used to only use Pandora, but the ads and not being able to listen to what I wanted to listen to at that moment started to get to me (other than my Hall & Oates Pandora station - that is always perfect). I finally decided to pay for a subscription based service and I gotta say I'm loving it. (Using Rhapsody). Haven't seen MOG. Wonder how it's library compares to Rhapsody?
Check out stereomood.com and last.fm. I am also surprised that you didn't mention rdio.com!
Also to the above posters - I hadn't heard of rdio.com before either. Just signed up for a trial and installed to my phone. :)
I just signed up for MOG after a friend recommended it. I think it's worth the full price. I'm very much an "album listener" still (maybe I'm showing my age) and that's why I prefer MOG to Pandora.
After I learned about the Android Pandora app (which I wasn't using at the time) leaking all kinds of information to their advertising servicers, I stopped using them outright.
Umm, Slacker.com?
How is it not in the list?
i totally dig grooveshark.com
select the music you want. free. ad-free. create your own radio stations based off of a song, or listen to one of their channels.
perfection.
I second @Erik. Slacker is more diverse than Pandora without the sketchy hiphop --> Disney effect. Infinitely more customizable than Pandora as well. The ads are just a few seconds and easy to tune out!
I think Spotify is the best, I used it for around six months and it is definitely the best streaming service ever, not only for its interface but also the audio quality. It lacks some artists of my like but overall it is the best.
Grooveshark is the service I use now since the limited access Spotify has outside Europe. I paid for the yearly subscription and now I have access to my music not only at home but at work and my mobile devices. On the iPhone and Android I can make playlist available offline and works great, although the sound quality is not as good as in Spotify, or at least I perceibe it that way.
I switch off between Pandora and Grooveshark. Grooveshark when I'm actively listening, and Pandora when I need background music.
I've never heard of some of these services, though, so I'm eager to try some new ones out.
I like accuradio.com - it doesn't work quite the same way as pandora, but there is a huge mix of genres, sub-genres, sub-sub-genres so it really is just as customizable. It's especially great for Christmas music. And, there's an app!
How is Rhapsody not on this list?? They have the biggest catalog and have been around forever--plus available in the U.S. I've been a subscriber for years, and have it going all the time - on my Sonos at home, my phone in the car and my computer at work. And, unlike Pandora, you can hear any song you want, as often as you want without ads. Sometimes, they even get stuff early!
Slacker beats the pants off Pandora
Thanks for the suggestions, everyone! I'll use them all to make a followup post. Is it shameful to admit I'd never heard of Slacker or rdio.com? I'll check them out for sure.
i've put a great deal of time into using all of these services and mog.com is indeed the best. rdio, slacker both fall short. slacker does not offer on demand listening and rdio lacks all of mog's incredible passive listening and discovery features. also mog provides their entire catalog in 320kpps audio.
Grooveshark's pretty good :)
It's nice to see a side by side comparison, although some of these services are really like comparing apples to oranges. The beauty of all of this is that there is so much choice and such a plethora of options for discovering, enjoying, sharing, and creating music on the web. It's all just a matter of what you're looking for in the experience. Rdio should definitely be on the list, but it's not flawless, either. Spotify seems excellent so far, but much like iTunes, I won't likely invest the time in managing content that is needed to make the most of it. Lately, I've been using a new location-based music service called Raditaz to discover new music that fits the place or activity that I'm in the midst of. Sort of Pandora with a spin. Whatever suits you best, it's great that the options are constantly improving.