I'm listening right now to Oistrach playing the Szymanovski Violin concerto which is really beautiful but never mentioned. Similarly the Bartok Violin concerti. There is also a violin concerto by Halvorsen (Maria Duenas recorded it recently) of the Handel Passacaglia arrangement, and another Norvegian composer, Borgstrom, all of which are worth a listen.
This is different from a large group of concertos which are not played very frequently but are quite well known (either as student concerto's like De Beriot, or considered second-tier like Vieuxtemps, Wieniawski, Glazunov, Khatchaturian, Spohr).
This got me thinking about why they are not so well known. Some of it may be that they are just not as good as the 'big' concerti, or the idiom is too modern for the general taste. I do recall an interview with a famous soloist (can't recall who, might be Hilary Hahn) who mentioned a few concerto's that are really beautiful but aren't played much, and she mentioned one (Dohnanyi maybe?) that has one extremely difficult passage which is the reason that soloists don't like it.
Maybe others have some further insights? Or know other concerto's that are rarely mentioned ?
(edited to add: I restricted it to non-contemporary composers (say before 1950) as contemporary music as a whole mostly doesn't have the general appeal that older music has, and I didn't want to let that topic derail the discussion)