The Capitoline Wolf breastfeeding Romulus and Remus
Hoshi Sato covering her breasts in 2152
Quark, as Lumba, with cosmetically added breasts
The breasts are located on the upper chest of many humanoid species and were used in breast feeding. (TNG: "A Matter Of Honor"; VOY: "Deadlock")
The protrusion of a man's breast was mentioned in A British Tar. (Star Trek: Insurrection)
"His eyes should flash,
and his breast protrude,
And this should be his customary attitude."
When the Enterprise NX-01 was captured and impounded by Suliban Cabal, the crew used their room intercoms to formulate a plan to escape. However, the only escape route was through the air vents and Ensign Hoshi Sato was small enough to fit. When she made her way to the outside of Lieutenant Malcolm Reed's room, her shirt was caught on a hook and she tried to get free, but ended up losing her shirt while sliding down. When she opened Reed's door, she held her hands over her breasts and told him to not say anything and to fetch her a shirt. (ENT: "Shockwave, Part II")
Jonathan Archer accidentally used the term "breast" while receiving a report from T'Pol. (ENT: "A Night in Sickbay")
Audrid Dax (in Quark's body during Jadzia's zhian'tara) thought there was "nothing quite like" holding a baby to one's breast. (DS9: "Facets")
In 2370, Pel covered her breasts to avoid being exposed as a female. (DS9: "Rules of Acquisition")
Quark himself had breasts cosmetically added in 2374 while posing as Lumba. (DS9: "Profit and Lace")
While on the Ba'ku planet, Deanna Troi asked Beverly Crusher if she had noticed how her "boobs" had started to firm up, as a result of the metaphasic radiation that affects that planet, to which Crusher replied, "Not that we care about such things in this day and age." - though Troi's snickering reaction suggested otherwise. Data, who overheard this conversation, repeated it to Worf. (Star Trek: Insurrection)