VVVVVV is a 2D platformer released in 2010 by Terry Cavanagh. It's a fun game with simple but engaging mechanics and the soundtrack is great.
The source code of the game was released in January 2020 (Announcement, GitHub repo). It's not free software, but the license allows anyone to compile the game and run it. The assets are proprietary though, but they can be downloaded here.
It's a fun game. You just need to compile it yourself, which should be easy for you.
Some quick instructions for a debian-like OS:
sudo apt install libsdl2-dev libsdl2-mixer-dev
git clone https://github.com/TerryCavanagh/vvvvvv
cd VVVVVV/desktop_version
mkdir build
cd build
cmake ..
make
wget https://thelettervsixtim.es/makeandplay/data.zip
./VVVVVV(More instructions in the GitHub repository)
Since the code was public, I decided to look inside, and eventually to make a small mod (link).
My mod adds portal mechanics using the mouse. Left and right click create blue and orange portals, and the player can teleport between the two.
Inverting gravity is still possible. When you combine it with the portals, it made the game too easy. To keep the game interesting, there are small modifications on a lot of levels.
Taking inspiration from hardcoded levels in the original source, I hardcoded the portals sprites. This was actually useful because I didn't have to modify the building process to include a new asset.
It was a fun project that was only possible because VVVVVV exists and its code was released, so thanks Terry.
Should you wish to play Portal VVVVVV, the building process is the same as the original game. I hope you enjoy it.
By the way, "Piercing the Sky" is the best track of the OST. It's not included in the original levels for some reason.