Here's a list of some Android apps I have found helpful. See also: Apps, Programs, Operating Systems, Phones.
Cogi – Notes & Voice Recorder
Record voice/surroundings (not calls). Did not record an important discussion.
Easy Voice Recorder
Record audio.
Developer: Digipom Inc. 1200 Avenue, McGill College Suite, 1100 Montreal, Quebec
Shazam
Recognize music.
Developer: Apple (previously Shazam).
Spotify
Play music from Spotify.
Track ID
Recognize music. You can use this without logging in to Facebook to share a music track via email. Came with Sony XPERIA Tipo. Discontinued approximately September 2018 (see Shazam).
Codeanywhere
Not very useful, but a nice idea.
Sand IDE Pro for Java
Java IDE. Could be useful, but I prefer desktop IDEs.
Developer: jimmychen812 @gmail.com.
File Manager Pro
See your files on your phone or tablet. Use Bluetooth transfer.
Google Play, Google Play (free), Web
BombSquad
Use the "Gather" button and then select "Local Network".
Developer: Eric Froemling.
OS Monitor
Show processes, connections.
Developer: eolwral (Gmail, GitHub).
Ping
Speedtest
NetX - Network Discovery Tools
See other devices on the same Wi-Fi network.
Developer: NetGEL.
Simple UDP Tester
Send UDP message to a PC application.
Developer: Lucus Darnell / Prodigy Productions, LLC.
ColorNote Notepad Notes
Developer: Social & Mobile, Inc. http://blog.socialnmobile.com. notesupport @socialnmobile.com.
Adobe Acrobat Reader
Developer: Adobe. adobereader-android @adobe.com.
Xodo PDF Reader & Editor
Developer: Xodo Technologies Inc. support @xodo.com.
QR Code Reader
Developer: Scan.me. support @scan.me.
Aggregator
Developer: Ștefan Șelariu, aggregator-support @tughi.com.
Stopwatch & Timer
Developer: stopwatchtimers.com. support @jupiterapps.com.
Google Play, Free Version, Web, Old Web Site
Wifi Analyzer
Developer: farproc.
Last modified: February 16th, 2019
Author: Tomi Häsä (tomi.hasa@gmail.com)
URL: http://sites.google.com/site/tomihasa/useful-android-apps