Unity
Unity – free if you are making less than $100K
https://unity.com/
Remember: some of the best features are in the packages, to be installed separately
https://docs.unity3d.com/Manual/pack-safe.html
https://docs.unity3d.com/Manual/pack-preview.html
Unity Asset Store – 3rd party assets marketplace
https://assetstore.unity.com/
Gold player – FP controls for unity
https://github.com/Hertzole/gold-player
Xinput – Game controller tester
https://thatsmytrunks.itch.io/xinput-controller-tester
Oikospiel Tools – Add basic interactions without coding
https://github.com/fernandoramallo/oikospiel-tools
iTween – tweening library for procedural animation
http://www.pixelplacement.com/itween/index.php
Ink – Unity integration
https://assetstore.unity.com/packages/tools/integration/ink-unity-integration-60055
Microsplat – more advanced alternative to Unity terrain
https://assetstore.unity.com/packages/tools/terrain/microsplat-96478
UCLA Mesh creator – extrude sprites in unity
https://assetstore.unity.com/packages/tools/sprite-management/ucla-mesh-creator-11066
Writing / Prototyping
Bitsy – make minimalistic exploration games
https://ledoux.itch.io/bitsy
Inky – hypertext editor for ink
https://github.com/inkle/inky/releases/tag/0.12.0
3D
Blender – the leading 3d creation suite
https://www.blender.org/
Sprytile – 3d tiles in blender
https://jeiel.itch.io/sprytile
Magikavoxel – easy voxel modeling
https://ephtracy.github.io/
Makehuman – realistic human generator
http://www.makehumancommunity.org/
AwesomeBump – generates normal, height, etc maps from a single image
https://github.com/kmkolasinski/AwesomeBump
Meshmixer – mash up 3d models (warning: models may be too high poly)
https://www.meshmixer.com/
Meshlab – Remeshing tool, especially to simplify and clean up photogrammetry models
https://www.meshlab.net/
Instant Meshes – Create a clean mesh from a high poly model
https://github.com/wjakob/instant-meshes
Terrain Party – height maps from real world terrain
http://terrain.party/
Snappy Tree – Procedurally generate trees (not optimized)
http://www.snappytree.com/
2D
Piskel – pixel art drawing and animation
https://www.piskelapp.com/
Pixelator – images to pixel art
https://ronenness.itch.io/pixelator
Paint of Persia – pixel rotoscoping
https://dunin.itch.io/ptop
TileSetter – tile set creator
https://www.tilesetter.org/
Tiled – map editor
https://www.mapeditor.org/
Ogmo – 2D level editor
https://ogmo-editor-3.github.io/
Music and sound
Chiptone – Generate sound effects
https://sfbgames.itch.io/chiptone
Bosca Ceoli – Simple sequencer, exports midi and wav
https://terrycavanagh.itch.io/bosca-ceoil
Bleeper – Generate single chiptune tones
https://pixwlk.itch.io/bleeper
Blender – Quick sound effects from samples
https://beta.blendwave.net/
Paul stretch – extreme sound stretching
http://hypermammut.sourceforge.net/paulstretch/