Join-Lemmy.org instance list - Official instance list.

Lemmy Explorer - Nice list of all instances with sorting and filters.

Fedidb instance list - Much faster to load & browse than Lemmy Explorer, but less options for sorting and filtering. Great if you just wanna check the top few instances quickly

Fediverse Observer map - Shows where all the Lemmy servers are physically located

Fediverse Observer list - Probably my least favourite of the options I know about, but it does exist. Fedidb and Lemmy Explorer are better.