The Trickster was the first: he discovered he had a 12-year-old son by an ex-girlfriend. (Neither Billy nor his mother has appeared since this story, which was part of 1998’s “New Year’s Evil” event.)
The Weather Wizard was next. Early in his run on the book, Geoff Johns introduced an ex-girlfriend of Wally’s who had an infant son with lightning-based powers. He turned out to be the result of a rebound one-night stand with the Weather Wizard (in his civilian ID, of course) just after she and Wally had broken up.
And now, with Identity Crisis, Captain Boomerang has a long-lost adult or teenaged son… who looks like he’ll be taking over the “family business.” Who his mother is remains to be revealed, though the next issue of either IC or The Flash seems likely.