If you've got any type of iDevice, than you should already be intimately familiar with Apple's all-in-one media player, marketplace and device syncing tool. iTunes tries to be everything that you'll ever need for media consumption rolled into one package, which some users appreciate, and others consider feature and resource bloat. Library management and marketplace features allow you to acquire and organize the material you want, and the seamless syncing make iTunes a must if you have one of Apple's various iDevices.
Foobar 2000 is an extremely modular freeware music player and library management app that comes with a surprising amount of software muscle behind its relatively bland interface. Out of the box, foobar supports supports a variety of formats such as MP3, MP4, FLAC and WAV, gapless playback, and media library features such as tag editing. In addition, Foobar 2000 is highly extensible, with a large selection of components you can add to the program such as DTS and AC3 support, multipoint room audio compensation, and even tree-style views of your media library with SQL support. If you're just looking for a quick-fire music player, there may be better options, but Foobar's customizability definitely rewards those who like to tinker with their apps.
Clementine is a versatile open-source music player and library management tool that's equally at home playing music in your hard drive or songs in the cloud. Covering more than just the basics of playback, Clementine includes a tag editor, cover art and queue manager, support for smart and dynamic playlists as well as a variety of playlist formats. In addition to local file playback, Clementine supports a variety of internet radio and music services such as Spotify and Soundcloud, as well as cloud playback from Box, Dropbox, Drive and OneDrive.
Already have a large collection of material over at iTunes and you want to get it onto your Android device? Try iSyncr, a desktop and Android app by JRT Studio (makers of Rocket Player). iSyncr plugs in to iTunes and then allows you to sync your music straight to your Android smartphone or tablet, allowing you to put iTunes content onto an Android device. Wired and WiFi syncing is available, and it can also import media from your device to your computer. It will set you back around $4.99, but that's certainly not breaking the bank.
Another music manager designed to take the sting out of migrating to an Android device from iOS is DoubleTwist Sync, an app designed to sync your iTunes music library to your Android device through WiFi or USB. DoubleTwist automatically reads the contents from your iTunes or Windows Media Player library and syncs it over to your Android device, along with photos and videos. DoubleTwist Sync is also designed to work both ways, so that playlist changes made on your device are reflected in your desktop library.
MediaMonkey is a popular media player and organizer reminiscent of iTunes and it's packed with tools to organize and edit your media collection. It plays music and videos and features powerful tag and metadata editing tools, album art downloading, media transcoding, syncing capability for Android and iOS devices, and UPnP/DLNA sharing. Many more advanced features such as automatic library management and on-the-fly transcoding auto-leveling volume are hidden behind a paywall, but what you get for free is already pretty impressive.
Another excellent total music management program is MusicBee, which is a great multifunction program for your media playing and library management needs. Import material from iTunes, Windows Media Player and easy manual and automatic tagging lets you quickly make sense of your music collection. Numerous library views and filters let you home in quickly on the content you want. Android device syncing lets you put your music onto your phone quickly. MusicBee also includes audio conversion tools, automatic song data retrieval, smart playlist creation, and more.