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