Given the ubiquity of Android phones, it's hard to believe there was once a time when the Google Play Store -- then called Android Market -- was nothing more than a side project few people knew about.

In fact, the store was still called Android Market as recently as 2012. What's perhaps even more remarkable is that the Android Market still exists in the Google Play Store. The caveat? You'll need a phone running Android 3.0 or earlier to download and install it.

But just because the Google Play Store is now a media behemoth, ready to sell you everything from apps to ebooks, it doesn't mean alternative marketplaces aren't available. Amazon Appstore and F-Droid are two of the best.

1. Amazon Appstore

With an estimated 600,000 apps, the Amazon Appstore is the largest distributor of Android apps outside of the official Google Play Store.

The store has gained a huge following thanks to its free content. If you install it, you'll find hundreds of apps, games, and in-app items that you can download without charge. The same content would cost you more than $20,000 on the Google Play Store.

The Amazon Appstore's biggest downside its lack of Google Play Services. Play Services underpin everything from user authentication to data syncing. It means you'll need to keep the Google Play Store on your phone regardless.

The other big drawback is the installation process. You need to alter your phone's security settings and sideload an APK file Amazon emails to you.

2. F-Droid

There are lots of other apps I could mention. SlideME and GetJar both deserve a shoutout. However, for something a bit different, check out F-Droid.

F-Droid is a catalog of "Free and Open Source Software" (FOSS). Unlike the Amazon Appstore, it's home to lots of apps that never make it to the Google Play Store, either because they perform illicit activities or they don't satisfy Google's stricter terms of service.

Because it's open source, the store is popular among hobbyist programmers and coders. Every app on the site has its source code readily available to download and use.

Which alternative Android app marketplaces do you use to download apps? Let us know in the comments below.