Open source projects are amongst the most interesting time sinks. Groups of people share a common goal and often volunteer their time to create a software product that significantly surpasses most paid software in terms of overall quality. Many receive donations, help from corporations, or at least glory and fame in the wider programming community. Some projects, however, fly under the radar of many.
Don't let youtube-dl fool you - it isn't merely a YouTube video downloader. It supports hundreds of websites and is extendible to support everything under the sun. It boasts plenty of useful options to cover even the most unusual of edge cases. Development progresses at impressive speed given the small number of maintainers, who are flooded with eager contributors who want to add their favourite site. If you ever wanted to download a piece of media off the web, youtube-dl is most likely going to solve your problems.
spacemacs is an emacs "distribution" (yes it is an operating system) which attempts to resolve the holy editor wars by taking the best of both worlds and reaching a middle ground. It is highly opinionated, with many plugins installed and pre-configured to enable vim-keybindings, IDE-like features for many programming languages, easily accessible self-documentation and so on.
With ancient but "tried and true" video players like VLC steadily declining in popularity, mpv is becoming more and more popular. The minimalist interface is a breath of fresh air. It features bazillion options to unleash your inner tweaker, like hardware decoding, playing over networking and local protocols, rendering options for upscaling, debanding etc. and rich plugin support through Lua scripting.
This is the tool for media black magic. Although it does receive great support from massive companies that use it, such as Google, ffmpeg often remains unnamed in the open source community. This is unfortunate, because it is singlehandedly responsible for allowing Linux distributions to play back any media files at all - your web browser uses it, video and audio players rarely do any decoding without it, and awesome artistic tools like Blender enable developers and users to not worry about the technicalities of multimedia. It supports more codecs than anyone can list and likewise every container you could hope for. Have a file with one video, 5 audio, and 30 subtitles, and you want to trim some of them out, reencoding to codecs of your choice, converting from 5.1 to stereo, and making the video upside down? No problem.