There's an amazon thread happening on reddit right now called, What free things online should everyone take advantage of?  And the user /u/njdeatheater put together this awesome list below.

3D modeling

• https://www.blender.org/
• /u/M4nusky says: www.tinkercad.com for very easy 3D modeling.
• /u/JCthulhuM says: http://www.sketchup.com/products/sketchup-make
• /u/BlenderGuy shares Free Online CAD Place: OnShape https://www.onshape.com/
• /u/danbo1221 says: For 3D CAD, you may want to consider AutoDesk Fusion 360 .
• For 2D, I recommend Dassault DraftSight
• For 2D Animation /u/DarkLinkXXXX says : Don't forget OpenToonz and Synfig for 2d animation.

Digital Painting:

• https://krita.org/
• /u/MudkipOverlord says CtrlPaint: http://www.ctrlpaint.com
• /u/Fiishbait says : Terragen has a free version of its amazing software, with only some restrictions on image render size, quality levels, but can still create stunning images.

Vector Drawing:

• https://inkscape.org/

Digital Music Production:

• https://lmms.io/
• /u/HP40 says: https://www.audiotool.com/

Music Mixing:

• http://mixxx.org/
• From /u/TaTayou :For digital music production there is also reaper, you can use the demo (which is like the full version) as long as you want(and not only during 60days as advertised). : www.reaper.fm
• /u/just_not_ready says: Audacity

Photo/Image Editing:

• http://www.gimp.org/
• /u/TunaLobster adds: Paint.Net
• /u/Comrade_Bender adds: https://pixlr.com/editor/

Go to school and take courses:

• https://www.edx.org/
• https://www.khanacademy.org/
• /u/Kong_Dong and /u/zzgoogleplexzz say: http://www.coursera.org/ - I knew I was forgetting one!
• /u/starlinguk adds: Future Learn

Learn programming:

• https://www.udacity.com/
• https://www.codecademy.com/
• /u/TricksterofU says: http://cscircles.cemc.uwaterloo.ca/
• /u/GSRoTu says: http://scratch.mit.edu/
• /u/PaulJP says : Visual Studio Community is free, and is basically pro without enterprise-level stuff: /. The MSDN (documentation) is free online ( https://msdn.microsoft.com ), as well as Stack Overflow for questions.
• /u/Hanta3 adds: Visual Studio with Monogame is great for getting started with object oriented programming.
• /u/CounterSeal says: http://www.theodinproject.com/
• /u/Stiino0 says: http://www.freecodecamp.com/
• /u/PM_ME_YOUR_IPv6 says: LearnCPP is a great source for learning C++ and some C
• /u/Fl1pzomg recommends:http://www.w3schools.com/

Learn about Cyber Security!

• /u/Pedigree_Dogfood says: https://www.cybrary.it

Livestream/Broadcast!

• /u/cormTiger says:https://obsproject.com/

Make a game:

• http://unity3d.com/get-unity
• https://www.unrealengine.com/
• /u/HeavyHDx says: http://www.godotengine.org/
• /u/zxj4k3xz says: http://www.yoyogames.com/get
• /u/thug-gamer says : https://aws.amazon.com/lumberyard/
• https://play0ad.com/ - free, open-source, historical RTS game, being built, and could use more help
• /u/ViKomprenas says: http://superpowers-html5.com/index.en.html

Learn a language:

• https://www.duolingo.com/ (App is free for iOS and Android)
• http://readlang.com/ - I just remembered this one, it's Chrome extension which translates to teach.
• /u/Ihmed says: https://www.memrise.com/
• /u/jjanpi says : https://lingvist.io/
• /u/wakawakafoobar says : https://www.clozemaster.com/

Learn Morse Code!

• /u/Vetrol says: https://www.morsecode.io
• /u/shampoocell says : http://www.lcwo.net Play old arcade games:
• https://archive.org/details/internetarcade
• Play old PC Games: https://archive.org/details/classicpcgames

Learn how to fly a flight sim:

• /u/FlightGearLego says: http://www.flightgear.org/ - Free Flight Sim

Nothing Needed

• Write a book or short story. No Internet needed. Just word and your imagination.

/u/bumbletowne shares this huge list of art stuff. :

• MORE ART! (shameless plug for /r/drawing and /r/learnart )

Also for digital painting:

• http://www.ctrlpaint.com/

And for regular painting/figure drawing:

• http://www.proko.com/
• https://www.drawspace.com/
• http://www.drawmixpaint.com/
• /u/mdhe adds: http://drawabox.com and its subreddit /r/artfundamentals for drawing.

(New Masters Academy-they have gesture practice there too)

https://www.youtube.com/channel/UCliUF1c8m7MUspaCykJljSg

To practice gesture:

• http://artists.pixelovely.com/practice-tools/figure-drawing/
• http://www.quickposes.com/

Reference libraries for people:

• http://characterdesigns.com/index.php?sitepage=photosets
• http://senshistock.deviantart.com/gallery/?offset=0

Palette selector (A THING THAT HELPS YOU PICK COLORS THAT GO GOOD TOGETHER)

• http://paletton.com/#uid=1000u0kllllaFw0g0qFqFg0w0aF

Can't think of what to draw? Here's an image randomizer of things to draw

• http://something-to-draw.com/