If you're using a standard Android tablet, you can use a utility such as DoubleTwist to get fairly close to iTunes' capabilities (it even works with iTunes libraries), but it doesn't work with the Nexus 7 unless you buy the $10 AirTwist add-on to DoubleTwist. DoubleTwist, with or without AirTwist, isn't available for the custom versions of Android that Amazon and B&N have on their media tablets, so you'll need to use a direct USB connection to transfer your computer's existing media (in OS X, you also need Google's primitive Android File Transfer utility).
Note that the iPad Mini and Nexus 7 both support MP3 and AAC (.m4a) audio, MPEG-4 (.m4v and .mp4) video, and ePub and PDF files. You can convert several common video formats to compatible MPEG-4 versions using OS X's included QuickTime Player utility or via third-party utilities for Windows. The Kindle Fire HD supports all the same formats except ePub, meaning you can only read books in its proprietary Mobi file format. (The free open source Calibre app for OS X and Windows can convert ePubs to Mobi format.)
All three media tablets put transferred music in their music apps; on the Kindle Fire HD, be sure to switch to the Devices pane to see them. But they handle transferred videos and books differently:
- The iPad Mini puts all personal videos in the Movies pane in the Videos app. The Nexus 7 puts transferred video in the Play Video app's Personal Videos pane. The Kindle Fire HD doesn't put the videos in the Videos window at all; you have to go to the Kindle Fire's Apps view, then open the Personal Videos app to see your transferred videos. (The Kindle Fire's Videos window shows only videos purchased at Amazon.)
- For books, the iPad Mini puts ePubs and PDFs in their books apps. The Kindle Fire puts copied PDFs in its Docs window and Mobi books in its Books window, both in the Devices pane. The Nexus 7's Play Books app can't access copied books at all, though the Kindle app can if you place the Mobi files in in the Nexus 7's Kindle folder.
If you're willing to live without iTunes, Amazon has the broadest video and music libraries, followed by Google, then Microsoft. You can watch iTunes-purchased content only on an Apple device, just as you can play videos or music purchased from the Google, Barnes & Noble, or Microsoft media stores only on their respective devices.