Another new tool in this vein is Fedifinder. It works by scanning Twitter bios of accounts you follow for strings that look like Mastodon addresses: https://fedifinder.glitch.me/

5 years ago there was a neat tool called Mastodon Bridge that did what Twitodon says it does but much better, I think because it didn't require everyone opt in. It stopped working because of some change Twitter made to their API terms of use. https://github.com/mastodon/mastodon-bridge

There's also Moa Party, but it's so complicated I have never used it. https://moa.party/