Some true one-hit wonders are Friend and Lover's Reach Out of the Darkness, The Hondells's Little Honda, Zager and Evans' In the Year 2525, USA For Africa's We Are the World (probably one of the best known one-hit wonders), and M's Pop Muzic.
Many others mentioned above are actually two-hit wonders. So often a group has one hit LP, one song of which is a big hit and another is a minor hit. Looking Glass did that, with Brandy the big hit and Jimmy Loves Mary Anne the minor hit. Still others had a hit, then had another several years later (Sugarloaf had Green Eyed Lady in 1970, and then had Don't Call Us We'll Call You in 1975. These are often confused with one-hit wonders (I classify that as minor mistakes since even the big labels that market them as one-hit wonders usually miss their other hit).