https://fedi.buzz is one for hashtags (might break soon) https://feditrends.com/ is one for articles https://mastodon.social/@TrendingBot <- extremely useful if you are on a small hosted instance

theres also https://fedi.directory/ that provides info

the biggest thing when it comes to the fediverse though is that it’s absent of an algorithm. thats the intention. you populate the content you see. so the key is to follow every single hashtag that interests you and itll start to populate it for you. if you are on a smaller instance there are also a.gup.pe groups which collect stuff for you