It has already been pointed out that Suetonius reported the words "καὶ σύ, τέκνον" as Caesar's last, and that these are the ultimate origin of Shakespeare's "Et tu, Brute?" (though it seems to be Richard Edes who first coined the Latin phrase, in his 1582 Latin-language play Caesar Interfectes, seventeen years before Shakespeare's Julius Caesar).
The literal meaning is clear, but for the sake of completeness I want to mention the other interpretation, which has become somewhat popular in recent years, especially since Kathryn Tempest drew attention to it in her 2017 book Brutus: The Noble Conspirator.
Apparently the archaeologist James Russell pointed out, in a 1980 article that I haven't been able to track down, that the words "ΚΑΙ ϹΥ" (that is, "καὶ σύ" in caps with a lunate sigma) frequently appear on curse tablets, mosaics, and other forms of art, often in an apotropaic capacity. Here, for instance, is a prominent 2nd-century example from the House of the Evil Eye in Antioch-on-the-Orontes, which shows the Evil Eye being attacked by animals and weapons with the words:
In this light, Caesar's last words possibly weren't a pathetic disbelieving "even you, son?", but rather a defiant (in Tempest's words) "See you in hell, punk".
The advantage of this view is that the preëxistence of "καὶ σύ" as a formula also provides an explanation for why Caesar would switch to Greek for no apparent reason—educated Romans of the era knew Greek, obviously, but they still spoke Latin in their day-to-day lives.
I don't know if this hypothesis has quite reached mainstream level yet, but I see no obvious problems with it, and it's not surprising it would largely have been missed so far.