Meld is a visual diff and merge tool targeted at developers

I use meld by default. Honestly it's not great. I kind of miss beyondcompare. It is however readily available and works well enough.

I used it a lot many years ago, not so much now. I don't even know if it's on my laptop. I'm not using any other alternative. Maybe git merge and git diff are good enough for my needs. I'm also using both gitk and gitg, usually gitg. Maybe they qualify as alternatives.

It's my favorite and default diff viewer.

I remember at one point there was a new version (based on gtk3 perhaps) that didn't work as well (useless type of scrollbars and slow) and I did some shenanigans to have an older version again on archlinux. However now even fresh installs do look and work good, I'm not sure what happened to that new version, maybe they improved the gtk3 based one to be as good as the one before it and I don't notice the difference anymore...

It's not open source, and comes with all the baggage of perforce, but p4merge and p4diff are just excellent tools that I install on every machine I work on.

How is it not open source? The very first download link is to download the source. And it’s GPLv2 licensed.