I'd suggest you really have two separate categories here @themarkymark, since "underappreciated" is a pretty open-ended idea.
One is the "established and possibly well-known, but don't enjoy the success, recognition or following commensurate with their efforts and contributions" and the other would be the (for any number of reasons) "largely undiscovered" authors/content creators, who could be new or old.
In the first category, I'd suggest @janton whom someone else mentioned, too... creates high quality original and interesting content. I also recommend @simplymike who does an enormous amount of work helping newcomers while also publishing quality content, all for very modest rewards... even though she often gets 200-300 votes per post, they are all from small accounts. @themanwithnoname creates consistently and has for a long time, and still lives in relative obscurity. Somewhat better known but still undervalued — given the amount of effort she puts into community building — would be @jaynie, even after 2+ years and 1000+ well made original posts she makes less than $2 on many of them.
On the more "undiscovered, but unique" front, I like @krazzytrukker, the blog of a long-haul trucker traveling the US with cats in the cab; artist @lucydyer who shares all original work...
Very good points @denmarkguy
Not sure if I said ** Thank You.!!** for this.??
Power Scrolling with tired eyes in the limited time I do get on here, leaves me missing great mentions like this. Sorry if so....
Seems my life is a perpetual "Cat-Chup" and we are always running behind......
"Catch-up" hmmmm...???
Might be a good title 4 our cuturday post...?
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