Image Editing

Pixlr – Fully functional alternative to Adobe Photoshop. (Web, Windows, Mac)
GIMP – Another Photoshop alternative. (Linux, Mac)
Paint.net – GIMP for Windows (Windows)
Seashore – Basic image editing with layers, textures, gradients and more. (Mac)
ImageOptim – Image utility that compresses your images for the web (Mac)
RIOT – Batch image compression for Windows (Windows)
InkScape – Professional vector graphics editor, replaces Adobe Illustrator (Windows, Mac, Linux)
Pinta – Another great Illustrator alternative. (Windows, Mac, Linux)
Canva – Makes design ridiculously easy with templates for infographics, banners, newsletters, and more. (Freemium, web, iPad)
Pixlr-o-Matic – An app with fun, quick and easy effects that is a joy to use (Web, Windows, Mac, Chrome, iOS/Android)
Photoscape – Fun and easy photo editing software. (Windows, Mac)

Photography

Polarr – Great alternative for Adobe Lightroom. (Web)
Shotwell – Photo manager with powerful organizational and RAW support features (Linux)
Picasa – Google’s photo organizer and editor is actually great, if a little bloated (Everywhere)
RawTherapee – Complete solution for all your RAW needs. (Windows, Mac)
FSpot – Full-featured personal photo management system (Linux)
FastStone Image viewer – Fast, stable and user friendly image browser with basic editing features
Irfanview – Lightweight image viewer and editor with batch processing abilities.

  Video

Playback & Entertainment
VLC – The video editor everyone loves. Super fast, plays almost every video format, and has many other hidden features – from video format compression to streaming podcasts. (Windows, Mac, Linux, Android, iOS)
Pluto TV – Forget Netflix and cable, Pluto TV is your new best friend. With over a hundred channels – from Cats 24/7 to Science to Baseball, Pluto TV combines the variety of web content with the leanback experience of TV. (Everywhere)
(Disclosure: I currently work at Pluto TV)
Miro – Just like VLC and also open source, with a beautiful interface. (Windows, Mac, Linux)
Kodi (formerly XBMC) – Your personal home theater. (Windows, Mac, Linux, Android, iOS)

Editing
OpenShot – Fully featured video editor, alternative to Final Cut Pro/Premiere Pro. (Linux)
Lightwork – Professional grade video editor (Freemium, Windows, Mac, Linux)
Miro – Video convertor that converts almost any video format
Blender – Powerful video and animation editor with exhaustive documentation.
Handbrake – Indispensable tool that can convert and compress any format (Windows, Mac, Linux)

 Productivity

Productivity Tools
Wunderlist – The best task-management app ever, period. (Everywhere)
Pocket – Found an awesome article but don’t have time to read it? Add it to Pocket with one click and read it later. (Everywhere)
OneNote – Microsoft’s free and powerful note manager (Windows/Mac/Android/IOS)
Evernote – A must have at this point, Evernote is your personal-everything manager. (Everywhere)
Trello – Project management software that will supercharge your productivity, (Web, iOS, Android, Win 8)
Pomodoro One – A free time management tool using the famous Pomodoro technique. (Mac)
Howl Timer – A time management tool with customizable time slots.
Sunrise Calendar – Beautifully designed with numerous integrations (including Google Calendar and Evernote). (Mac, iOS, Android)

Office Software
Google Docs – A complete alternative to Microsoft Office in the cloud. Say goodbye to crashes and lost unsaved documents. (Web)
LibreOffice – Downloadable alternative to MS Office and Google Docs (Windows, Mac, Linux)
Atom.io – Hackable text editor from our friends at Github. (Windows, Mac, Linux)
Sublime Text – The best text editor ever. (Freemium, Windows, Mac, Linux)
Foxit PDF Reader – Great alternative to Adobe Acrobat Pro. Create, convert, and edit PDFs. (Windows, Linux)

Messaging

Slack – Say goodbye to email. Slack is a one-stop solution to all your messaging needs in and out of the workplace.
Adium – Cross platform IM client.
Pidgin – The ‘universal chat client’. Works with everything from Google Talk to IRC.
Telegram – Alternative to WhatsApp. (Android, iOS, Windows Phone)
Skype – Free video and audio calling.
Viber – Just like Skype.
KarmaBox – A tiny utility that sites in your menubar and notifies you when you get new reddit mail. (Mac)

Music

fooBar – My favorite personal music player ever. Don’t be fooled by the not-too-pretty interace. foobar is the fastest, most lightweight and customizable music player available. (Windows)
Spotify – Ad-supported free music streaming. (Everywhere)
Audacity – Create, record, edit and remix music. Your personal home studio. (Windows, Mac, Linux)
Pandora – Like Spotify for radio.
Rdio Radio and on-demand streaming, available in more countries than Spotify and Pandora.

 Utilities

Everything Search – The fastest search for your computer, ever. Find anything in miliseconds. (Windows)
Flux – Protect your eyes by automatically adjusting your screen’s brightness according to the time of day. (Windows, Mac, Linux)
Dashlane – The best password manager ever made – it is an even better alternative than its many paid counterparts. Nothing is stored in the cloud and everything is encrypted – and the best part, it’s all cross-platform. (Windows, Mac, iOS, Android)
Ccleaner – The best tool to clean your computer, manage startup programs, and so much more. An essential. (Windows, Mac, Android)
BetterTouchTool – This will change your life. Open apps, close tabs, dock windows, delete files – all without lifting a finger from the touchpad. If you had to get only one app for the Mac, let it be this one. (Mac)
TreeSize – Scans your computer and visualizes exactly where and which apps are using up your disk space. (Windows)
DaisyDisk – Like TreeSize, except for Mac. (Mac)
Alfred – Search files and web, open apps and so much more. (Mac)
7-Zip – Zipping and unzipping software that is truly free – no nagging trial periods. (Windows)
The Unarchiver – 7-Zip for Mac. (Mac)
Calibre – Complete e-book manager, convertor and reader. (Windows, Mac, Linux)
App Cleaner – Completely uninstall apps and their hidden files with one click. (Mac)
Rocketdock – Get a Mac OSX-like dock for your Windows computer. (Windows)
Rainmeter – Completely customize and transform your desktop with skins, weather forecast, app launchers, quotes, and more. (Windows)
FileZilla – The best FTP client. (Windows, Mac)
Dropbox – The essential file backup solution. (Everywhere)
Memory Clean – Keep your RAM use under control with one click. (Mac)
coconutBattery – Track your Macbook’s battery health. (Mac)
Jing – Capture screencasts or screenshots and share them instantly. (Windows, Mac)
FastStone Capture – Image capture with a full-page (scrolling) screen capture feature. (Windows)