It's free and works on the more popular browsers, without a client, which is is why it survives despite the fact it steers like a cow.
Fantasy Grounds costs money and requires downloading a client. It's the primary alternative you hear about. No go if you're poor or have an OS they don't support.
Google turned up this thing: https://www.rptools.net/ ... It's free and while it requires downloading the tool, but it's a java client so it should run various systems, and they have a Discord for support. I haven't tried it so I can't vouch for it, but it sounds better than any alternative I've seen so far.
I mean, I assume you've been here, but I haven't heard much about any of these other tools.
-Kirt