Ever since Google bought Simplify Media and killed it for the iPhone, there has been a need for a reliable way to get your DRM-free music library to your iPhone. Audiogalaxy has done that, and quite a bit more. Audiogalaxy basically streams your music collection, using a helper app, to another computer, an iPad, iPhone, or an Android phone.
To get started, you sign up for free, and a Web-based client will scan your hard drive for your music collection. If you have a lot of music, it can take a lot of time, so be patient. When that's done, you can grab the free iPhone or iPad client, sign in, and your music is with you. The app works fine over 3G or Wi-Fi; audio quality is good, but it's not CD quality. I tried the app several times, and it worked well on a three-bar 3G connection. The app also supports background play in iOS 4.
Audiogalaxy has been out for quite some time, in various stages of beta. It still appears to be in beta, but in terms of streaming, I saw no problems.
On the other hand, some things about the app are just plain weird. It doesn't always follow the metadata rules, so any albums that I had with multiple artists were all listed as individual albums. That's not good, and it makes for a very cluttered list of music. The developers say a fix is coming (perhaps in the next version). The other thing I didn't like was that the app failed to capture my playlists, so while the albums are there, playlists are absent. I think this is because my music collection is on an external hard drive, and I'm hoping to see that fixed as well.
For those with long memories, the Audiogalaxy name may be familiar. In 1998, the company created an FTP site index called The Borg Search that allowed peer-to-peer sharing of music. The RIAA wasn't happy; eventually, there was a settlement and little was heard from Audiogalaxy. Now, Audiogalaxy is back and placeshifting your music to your devices.
For those who loved the Simplify Media app before it was assassinated, I think Audiogalaxy will fill the bill nicely. When some of the rough parts are ironed out, I think it will be a must-have app for your mobile devices.