Ecamm Network has released a Mac OS X application called iPhoneDrive which allows you to store arbitrary files onto your iPhone. The purpose appears to be a way to emulate "drive mode" with the iPhone, allowing you to use your iPhone as an expensive USB flash drive.
The software provides no access to other files on the iPhone, so you can't copy photos, music, movies that have previously been synced with iTunes.
The system requirements include Mac OS X 10.4.10 and iTunes v7.3 and costs $9.95 for the full version. A free week long demo is provided.