I was just looking at a list of Beatles songs, with UK and US chart positions. "Ob-La-Di, Ob-La-Da" was none UK, 49 US, and "A Day in the Life" was 63 UK, none US. That may not qualify either of them as unpopular, but I definitely think they both deserved higher chart positions. Two great songs, especially "A Day in the Life".
"A Day in the Life" is quite possibly the best Beatles song ever made. I also quite enjoy the never played on the radio "Run For Your Life" off of Rubber Soul
(which is quite possibly the best Beatles album- at the peak of their pop phase and while Lennon/McCartney were still doing a lot of the songwriting together, but just the point where you could really make out which songs were more Lennon and which more McCartney).
Pretty much the entire middle section of Nirvana's Nevermind
I think counts toward the OP. "Breed" never got much airplay, "Territorial ****ings" was a sweet song, "Lounge Act" was one of my favorites off the album, "Stay Away" was great- the best songs off the album never really got much play (which, considering how many times "Smells Like Teen Spirit" has been re-released, is probably a good thing). Incesticide
had some great nuggets, including three of the best cover songs ever recorded- Devo's "Turnaround", and the Vaselines "Molly's Lips" and "Son of a Gun". Off of In Utero
I always enjoy hearing "Scentless Apprentice", "Very Ape", and "Radio Friendly Unit Shifter" (the latter of which is probably my favorite off the album and Nirvana albums in general).