Like many people, I have a music library that's way too large to fit on my iPhone. (Yo, Apple! Can we please get a 64GB model already?) Fortunately, I can still listen to every track I own thanks to Simplify Music 2.0 (formerly Simplify Media).
The app streams tunes from your PC by way of a music-server program that's available for Windows, Mac, and Linux systems. Download it, install it, then choose the folders you want it to scan, monitor, and queue. (Alas, the program can't stream DRM-protected iTunes purchases, so it's time to move everything to iTunes Plus.)
The desktop software costs nothing. The Simplify Music iPhone app has an introductory price of $2.99, but the developer plans to bump it to $5.99 in a few weeks.
Once everything's installed and configured, just leave your system running and hit the road. When you run the app, you'll be able to browse your entire desktop music library by album, artist, genre, or song. Tap what you want and presto: the music plays, complete with album art and even song lyrics (which is so cool, it single-handedly justifies the cost of the app).
Want even more variety? Get some buddies on board: you can stream music from friends' music libraries in addition to your own.
Version 2.0 adds some nifty amenities, including a search function (something the iPhone itself doesn't have--yet), an add-to-favorites option for building an on-the-fly playlist, and a "scrub bar" for easy backward/forward scanning.
As you might expect, a Wi-Fi connection produces the best sound quality, though 3G runs it a close second when you have four or five bars. Even over EDGE, Simplify Music delivered surprisingly good sound, about on par with FM radio.
Though the app was sometimes annoyingly slow to connect to my PC, for the most part it worked like a charm. And much as I'm loathe to leave my system running 24-7, I'm now hopelessly addicted to having access to all my music.
Simplify Music makes that a reality, meaning I'll probably think twice about springing for a new iPhone just to get more memory. With the money I save, I'll be able to buy a lot more tunes.