Someone who just bought his first Mac asked me which essential (GUI) apps I currently use that I could recommend to him. As this will probably happen again I'll just keep this list here so I can easily link to it.

1Password

• Password management
• iOS sync, all my important documents / certificates are stored in there and sync with my phone and browser extensions
• 1Password also has a web interface so you don't have to install the program if you are on some other computer
• Web: https://1password.com/

AppCleaner

• Delete apps and clean up related files like preferences and other clutter
• Web: https://freemacsoft.net/appcleaner/

Arq

• Client side encrypted backups to a bunch of services, I use it for Amazon Drive, Google Drive and my own server via SCP.
• Web: https://www.arqbackup.com/

Atom

• Text editor for Go development with go-plus
• Web: https://atom.io/

BetterSnapTool

• Use it as a window manager (cmd + shift + arrow key to move window around the screen), couldn't live without that
• Web: https://itunes.apple.com/de/app/bettersnaptool/id417375580?mt=12

Calibre

• Converting eBooks and move them to Kindle, ugly but it's the best for this use case
• Web: https://calibre-ebook.com/

Captured

• Automatically upload screenshots to your own server and copying URL to clipboard
• Web: https://itunes.apple.com/at/app/captured/id414675451?l=en&mt=12

Carbon Copy Cloner

• Bootable Backups to an external HD on top of TimeMachine and Arq
• Web: https://bombich.com/

Charles

• mitm proxy capatable of mitm'ing SSL connections, very useful for reverse engineering APIs
• Web: https://www.charlesproxy.com/

Docker

• Docker.app is a very easy way to have the docker deamon running on your Mac
• Web: https://docs.docker.com/docker-for-mac/

Flux

• Screen is less bright at night so your eyes start to hurt...later
• Web: https://justgetflux.com/

iTerm

• Terminal replacement with great tmux support
• Web: https://www.iterm2.com/

MacDown

• Markdown editor with preview and syntax highlighting, you could also get this done in Atom though
• Web: http://macdown.uranusjr.com/

Paw

• REST client with a bunch of great features, especially the sync and a way to set variables in routes is super useful.
• Free alternative: Postman
• Web: https://paw.cloud/

Postico

• Postgres client
• Web: https://eggerapps.at/postico/

Pixelmator

• Image editing, has all the features I need and I don't have to use annoying Adobe things
• Web: http://www.pixelmator.com/mac/

Textual

• One of my most used apps and the best IRC client on macOS, lots of features, ZNC support, Tor support included
• Web: https://www.codeux.com/textual/

Tweetbot

• Twitter client without sponsored tweets and ads, supports multiple accounts, muting etc.
• Web: http://tapbots.com/tweetbot/mac/

Viscosity

• My favorite OpenVPN client because it doesn't look as ugly as Tunnelblick and does everything I need
• Web: https://www.sparklabs.com/viscosity/

Tower

• I like GUI git clients, sue me
• Web: https://www.git-tower.com/mac/

Transmission

• Torrent client, whitelisted on most trackers
• Web: https://transmissionbt.com/

Dash

• Offline documentation and snippets, I mostly use it for the snippets because it supports placeholders in snippets too
• Web: https://kapeli.com/dash

XLD

• Lossless audio decoder, I use this to convert Flac to Apple Lossless while automatically dropping this into the iTunes watch folder so it gets added automatically
• Web: http://tmkk.undo.jp/xld/index_e.html

LICEcap

• Screen recording with gif output
• Web: http://www.cockos.com/licecap/

TripMode

• Useful if you tether through your phone a lot while travelling, enable network access on a per application basis.
• Web: https://www.tripmode.ch/

NepTunes

• Scrobble to Last.FM from Apple Music / iTunes
• If you don't need Apple Music integration there's also QuickScrobbler as a free alternative
• Web: https://itunes.apple.com/us/app/neptunes-for-itunes-spotify/id1006739057?mt=12

GPGTools

• GPG encryption integration for Mail.app and GPG Keychain to manage your GPG keys (Mail integration not updated for Sierra yet)
• Web: https://gpgtools.org/

Kaleidoscope

• Diff client with great Tower integration
• Web: http://www.kaleidoscopeapp.com/

Yate

• Music tagger
• Web: https://2manyrobots.com/yate/

The Unarchiver

• Extract rar archives and a lot of other formats
• Web: https://itunes.apple.com/app/the-unarchiver/id425424353?mt=12&ls=1