They could've called this one Secret Origins of Off-Brand Super-Heroes, since the line-up of characters is so diverse and mostly B-List, but I think that's the fun of collections like these--for once, you're not stuck with reading how Jor-El and Lara sent baby Kal-El to Earth for four hundredth time.
"Birth of the Atom" by Gardner Fox, Gil Kane, and Murphy Anderson
"Battle of the Tiny Titans"(Part 2 of previous story) by Fox, Kane, and Anderson
"Swamp Thing" by(who else?) Len Wein and Berni Wrightson
"The Million-Dollar Debut of Batgirl" by Foz, Carmine Infantino, and Anderson (tho the CBDB says the inker was really Sid Greene, according to Julius Schwartz's records)
"The Kid From Atlantis" by Robert Benstein and Ramona Fradon
"The Origin of Green Lantern's Oath" by John Broome, Kane, and Anderson
"The Super Dog From Krypton" by Otto Binder and Curt Swan
"The Origin of Dr.Fate" by Fox and Howard Sherman
"Green Arrow's First Case" by Ed Heron and Jack Kirby
The Origin of Speedy in "The World's Worst Archer" by Bernstein d Lee Elias
...The Atom, Swamp Thing, Batgirl, Aqualad, Krypto, Dr.Fate, Green Arrow, and Speedy--now there's an assemblage of diverse superheroes! Wonder if they had anything to talk about at the photo shoot for the cover(btw--nice homage to the classic first JLA/JSA team-up by Andru and Giordano)?