This is a list of Alternative and Open Source Android Apps and OS I could recommend because I use most of them since November 2013 under Paranoid Android with a Nexus 5.
Most of these apps are available via F-Droid.
DiskUsage provides a way to find out which files and directories on the SD card of your Android phone are consuming the most space. It displays diagram with directories/subdirectories proportional to their size. Users are allowed to zoom in to look at specific directory content.
A fork of the last GPL version of EBookDroid. Supports the following formats:
Comic Books (cbz) (NO support for cbr (rar compressed))
FictionBook (fb2, fb2.zip)
Collaboration with electronic publication sites and access to online ebook catalogs is allowed by the supported OPDS protocol.
Kimai is a free open source timetracker. It tracks work time and prints out a summary of your activities on demand. Yearly, monthly, daily, by customer, by project, by action …
It’s simplicity is its strength. Due to Kimai’s web browser based interface it runs cross-platform. Likewise you can install it as a web service or as a single-user program on your local workstation.
NewPipe is a lightweight YouTube front-end for Android.
It doesn't need the YouTube-API or any Google Play Services, as NewPipe only parses the YouTube website in order to gain the information it needs.
Therefore this app can be used on devices without Google Services installed. Also, you don't need a YouTube account to use NewPipe, and it's FLOSS.
Display general information about a video
Watch YouTube videos
Listen to YouTube videos (experimental)
Select the streaming player to watch the video with
Download audio only
Open a video in Kodi
Show Next/Related videos
Search YouTube in a specific language
Watch age restricted material
Display general information about channels
TTRSS-Reader is a client application for the project Tiny Tiny RSS , a PHP-based online feedreader which runs on your own webspace. If you want to host your own feedreader, want a cross-device-solution and don't want to use Google Reader this is your app! TTRSS-Reader offers automatic update on startup, filtering articles by status and efficient browsing through categories and the virtual categories "Fresh", "Starred" and "Published Articles" as well as Labels which you can define by yourself. You can browse the articles offline when they have been synchronized before and the ImageCache can be used to pre-load all included images from the articles too.
wallabag is an opensource self hostable application for saving web pages.
wallabag extracts the article's content (and only its content!) and displays it in a comfortable view.
Moreover, wallabag is responsive: you can read your articles on your smartphone or your tablet.
Android, iOS or Windows Phone: wallabag application is available for your smartphone.
It's so easy to read your saved articles on your phone. And it also works when you're offline!
If you already have an account on Pocket©, Readability©, Instapaper© or even wallabag, you can import your data into wallabag.
wallabag provides a powerful tool to filter your saved articles. It's so easy to look for them.
wallabag is translated into more than ten languages. If yours is not yet available, please help translate.
All your data belongs to you. You can download all your articles in many formats: PDF, ePUB, .mobi, JSON, CSV, txt or HTML.
Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router. It turns you android phone into a handy Wi-Fi analyzer!
App Permissions (from Google Play)
Version 3.8.5 can access:
-modify or delete the contents of your USB storage
-test access to protected storage
-view Wi-Fi connections
-connect and disconnect from Wi-Fi
-prevent device from sleeping
-full network access
-view network connections
Zoiper IAX & SIP multilanguage and multiplatform (Windows, Linux and Mac OS X) softphone is a VoIP soft client, meant to work with any IP-based communications systems and infrastructure.
Zoiper Classic BIZ provides secure high-quality voice calls and conference, fax sending and receiving functionality, and enhanced IP-calling features wrapped in a compact interface and small download size.