At more than 10% of the population, the Copts are a sizable minority in Egypt, and in most towns church towers can be seen side by side with the mosque minarets.
The Coptic history is fascinating, stretching back to the evangelist Saint Mark, who travelled to Egypt to spread the Gospel and founded the Church of Alexandria in approximately 42 AD - making it one of the oldest churches in the Middle East, and certainly in Africa. (Later, the Coptic church would split off from the Roman Catholic Church after a long debate over what would seem to be theological hairsplitting, involving the divine vs. dual nature of Christ.)
The Copts' early presence in Egypt can be seen, for instance, in the graffiti they left in some of the Pharaonic tombs in the Valley of the Kings. (Again, check the Theban Mapping Project for images - particularly the tomb of Rameses IV.)
Another interesting piece of Coptic heritage is the Coptic script, an adaptation of Greek with the addition of six ancient Egyptian characters (from the Demotic, to be precise). Though nowadays only used in liturgic context, it can be said to retain the last trace of one of the oldest languages in the world: Egyptian.