Roger Bacon's diary is a diary given by Professor Quirrell to Harry Potter in October 1991. According to Quirrell, the diary belonged to 13th-century muggle philosopher Roger Bacon. This is a significant gift to Harry as Roger Bacon can be considered one of the earliest advocates of the scientific method.
According to Quirrell, Roger Bacon was a muggleborn wizard who got accepted to Hogwarts but refused to go and instead investigated magic by himself, recording his research into the diary.
See T. M. Riddle's Diary.