Who's This? The space cadet on page 9 of Who's Who vol.XXIV.
How you could have heard of him: Tommy was a featured player (more or less a villainous one) in Howard Chaykin's Twilight mini-series, and has had cameos whenever DC's space opera future has been evoked. The name turns up Stan Lee's Just Imagine series, Starman One Million says Tommy Tomorrow II was a prominent Starman, there's a Captain Tomorrow in the Trek/Legion crossover, and a Major Tomorrow of the Planeteers is referenced in Magog #7, but that's really the extent of Tommy's post-Crisis career. He's been an Easter Egg more than a character.
Example story: Action Comics #235 (1957) "The Interplanetary Joy Ride" by Otto Binder and Jim Mooney
Who else? Skipped over Tobias Whale, too well known as Black Lightning's only villain of note; Tokamak because he was part of Firestorm's greatest ever arc when I was a kid, so I can't bring myself to think of him as obscure; and Tomahawk's Rangers, who don't particularly interest me. Besides, our next stop will almost certainly feature the American Frontier.