Palm's Pre smartphone has received fairly positive press coverage for claiming to sync "seamlessly" with iTunes. The Boy Genius Report blog points out that free software can do pretty much the same thing with any removable drive.
iTunes Agent, a free Windows app, sits in the system tray and lets you turn anything that plugs in through a USB slot and can store things into an iTunes-friendly device. Plug in your device, create a "New" device in iTunes Agent's preferences, give it a name, and point to a folder where you want synchronised music to be kept, and the software creates a unique file to identify the device to itself and iTunes. Take note that it will wipe out any music manually placed in there already, but from then on out, iTunes Agent takes care of your music transfers from a single iTunes playlist and a system tray icon that you can click "Synchronize Now" from.
On Macs, The Boy Genius Report recommends iTuneMyWalkman for a similar experience to iTunes Agent, although that software actually looks more integrated and easier to manage than its Windows counterpart from the screenshots.
Some phones and MP3 players come with their own iTunes hook-ins, but if you've got another third-party solution to syncing non-Apple media players to iTunes, let us know in the comments. But, every phone can sync with iTunes ... [Boy Genius Report]