Ask HN: What is a good free alternative for Picasa for photo editing?

Now that Picasa is end-of-life, I would like suggestions about an alternative photo editor that is amateur-friendly. I am not a professional photographer. I am a programmer. But I do like to perform some quick enhancements on the photographs (like altering brightness, shadows, etc.). Is there a good free alternative (free as in free speech or free as in free beer)? If it works on all three of Mac, Linux and Windows, it's a bonus!