Allow me to plug a dictionary/thesaurus site I've been running for decades called OneLook (https://onelook.com). Although it's not specifically aimed at songwriters, it does attract many of them as users. Over the past thirty years, I've added all sorts of brainstorming features for creative writers—like the ability to search for words by description, to match words with a given meter, and more recently to discover which colors a word might evoke or vice versa.

(I’m also the creator of RhymeZone so I'll plug that too! I no longer operate it, but I can pass along any feature requests you might have to its new owners.)