Cogito ergo sum: "I think therefore I am", from René Descartes' Discourse on the Method.
Memento Mori: "Remember you will die" or "Remember your mortality", attributed to a Roman slave as a way of deflating his General master's ego.
Persona non grata: "An unwelcome person", which is self explanatory.
Argumentum ad hominem: "Argument to the man", refers to an insult or argument that addresses a human being. (Used in debate when a person is trying to disprove the truth of a claim by targeting the integrity of the speaker, rather than the claim itself. Basically, the meat of most internet arguments.)
sic.: Short for sic erat scriptum, meaning "Thus it was written", usually used in place following a quote in most formal documentation.
Carpe diem: "Seize the day", YOLO, for smart people.
Per alas porci?: "On a pig's wings?", short for ad astra per alas porci, meaning to the heavens on a pig's wings.
E pluribus unum: "Out of many, one", usually used to mean united as one, with all of a group working towards the good of that group, but in this case used completely literally: From the work of many, one gains.
Citius, altius, fortius: "Faster, higher, stronger", submitted by Daft Punk as the official Olympic motto following a short period of displacement in the time stream. During this period, Guy also boned Cleopatra.
Ad astra per aspera: "To the stars through difficulty", meaning to greatly gain from severe hardship.
Deus ex machina: "God from machine", a literary cop-out using an unprecedented, unforeseen, and often times unlikely method to solve a story's problems.

I saved a few google searches.