Android and iOS app discovery and search engines overview

Introduction

Recently a friend of mine asked how to check if an app or type of app with certain functionality already exists. He had a great idea for an app (he thinks :) so wanted to figure out: does such an app already exist?
Entering the name of an app is not sufficient of course, you want to search on offered functionality too, since the name of the app might not match the functionality. E.g Tinder is a dating app, but the name Tinder doesn't show that (at least not in its early days!).

I knew there are a couple of websites and apps that provide that, but had a hard time coming up with more than one. This list is pretty ok, but already partly out of date.
So I decided to create this blogpost with a list of the current recent websites and their apps and some(!) app-only search apps that provide app discovery.

Main focus of this post will be on app search engine websites, not apps! Mainly because the research for an existing app you'd probably like to do behind a computer (laptop or PC), not a phone or tablet, since it requires quite some typing. E.g all types of ways of describing the functionality.
Also most focus will be on Android, but also some iOS search engines will be mentioned.

Below is a list of app search engines I found, including a screenshot and my short take on it. If any search engine is missing, please leave a comment and I'll check it out. If appropriate I'll add it to this page!

App discovery and search engines

1 Standard stores

Of course the standard app stores have their own search engine, either online and/or on the platform itself, e.g:

Google Play Store for Android

Apple App Store for iOS

Amazon App Store for Android

There are quite some more 3rd party app stores like SlideMe, but usually these are considered less important than the "big ones", so chances are minimal that there's an app in those stores which is not in the big app stores. But for adult content you mighthave to check those.

2 AppBrain

URL: http://www.appbrain.com/
Platforms: Android
Has app: yes

3 Quixey

URL: https://www.quixey.com/
Platforms: Android, iOS, Windows, Blackberry
Has app: yes

4 App Annie

URL: http://www.appannie.com/
Platforms: Android, iOS, Amazon, Mac
Has app: yes but can't tell from the screenshots if it includes searching etc

This is more intended for app analytics and ASO (app store optimization) and SEO but still usuable for checking what type of apps exist, e.g by keyword.

App Annie recently purchased Distimo, another well known app statistics/analytics platform.

Most parts are for free, "enterprise" is not.

5 AppGravity

URL: http://appgravity.com/
Platforms: Android, iOS

Has app: yes

6 AndroLib

URL: http://www.androlib.com/
Platforms: Android
Has app: no

Still refers to "Android Market" instead of Google Play.

7 AppsZoom

URL: http://www.appszoom.com/
Platforms: Android, iOS

Has app: yes

8 MobileDevHQ (formerly known as AppStoreHQ)

URL: http://www.mobiledevhq.com/
Platforms: Android, iOS

Has app: no

Similar to AppAnnie more intended for ASO and analytics and keywords optimilization, but can still come in handy. 

9 Regular search engines

More and more regular search engines are optimizing for finding apps. For for example Google, Apple and Amazon it's of course in their own interest to help people find apps for their platform.

10 App only

There are also many apps out there trying to help you find apps. I did not investigate those, but a query you could run to find a list is this one. 

11 DIY

If none of the above work for you, you could consider writing your own search engine/crawling engine. Here's a script for the Google Play store.