Who's This? The guy in the tux on page 2 of Who's Who vol.XI.
How you could have heard of him: Aside from crowd scenes in Morrison's limbo and Forgotten Heroes flashbacks, there have been a couple of attempts at bringing back the Immortal Man. In 1991's Flash #48 and 49, Wally West meets a woman who may or may not be the Immortal Man's first female body (Doctor Who fans take note!). And there was some question as to whether the Resurrection Man had any link to this older character, proved false in a flashback in which Immortal Man was erased from existence in post-Crisis continuity. Most recently, he was seen as a member of the Society of Super-Heroes of Earth-20 in Final Crisis: Superman Beyond #1.
Example story: Strange Adventures #185 (1966)
If I were DC, I'd revamp this character, take out the super-powers, and develop it as a TV series with a rotating star. What. I've seen worse high-concept shows.
Who else? The Inferior Five have been mentioned, but they're way too infamous to truly be obscure. And yet, my next target IS a well-known character, except she isn't. FOLLOW THE CLUES NEXT WEEK!