Many of the best-known Latin quotations are from Horace, including carpe diem 'seize the day', dulce et decorum est 'it is sweet and fitting', in medias res 'in the middle of things', and nil desperandum 'keep your pecker up' (roughly).

Some more good ones:

Some other phrases, perhaps better known in English than in Latin these days, are also his: Homer nods (dormitat Homerus); the irritable tribe of poets (genus irritabile vatum).

He also coined the word sesquipedalian, literally a foot and a half long, for very long words.