VVVVVV, the clever platformer from Terry Cavanagh where you reverse gravity instead of jumping has now had the source code opened up so you can see the dirty innards.
The open license doesn't cover the assets (icons, art, graphics or music) which are still under a proprietary license. So you will need some to play with it, which Cavanagh said you can get from the Make and Play Edition for personal use and that edition also has the tools to make levels.
Why did they decide to do this though? Well, VVVVVV is just about ten years old and releasing it pretty much changed Cavanagh's life so they're doing it as a thank you to everyone who purchased it and supported them. Linux support came a little later, with the port arriving in 2011 as part of the third Humble Indie Bundle.
You can find it on GitHub which has two editions, one for desktop and one for mobile. It's not "open source" though, since the license has a few special restrictions with it aimed at non-commercial use. Find out more in the official announcement which goes over more details. Lovely to see more developers open up their code, I do wish more did this once a game is well past its prime to help others learn and to keep their games alive on future platforms after the original developer moves on.
More recently, Cavanagh also released the excellent Dicey Dungeons you definitely need to try.