MeloDroid let's you sync iTunes music to your Android device both wirelessly and over USB and use your Android device as a wireless remote for iTunes, and includes a ton of useful features. It's the ultimate iTunes sidekick. MeloDroid is comprised of two components.
MeloDroid.exe is a Windows program that acts as a server for the remote, and allows you to USB sync your Android phone with iTunes (including playlists). It's very easy to use and set up. MeloDroid has some useful features in addition to this, such as the ability to disable iTunes services that you don't use (like Bonjour and iTunesHelper) and display Now Playing song notifications. It even gives you the option to shut down iTunes automatically when you're not using it, and starts it back up when you are. The download is conveniently located on the Download page, and it's free!
Note: Windows only!
The MeloDroid app, now with QuickSync!
This is the app for Android. It let's you sync your iTunes music to your Android phone wirelessly, and serves as a remote control for iTunes. Some other cool features:
The volume buttons on your Android device control the volume in iTunes
iTunes pauses when you receive or make a phone call, and resumes when it ends
The word "The" is ignored when alphabetizing your artists, songs, and playlists
The interface is very straightforward and simple to use