From the Tyndale Bible Dictionary:
SILOAM, TOWER OF Edifice that collapsed, killing 18 people. Jesus compared those killed by the fallen tower to the rest of the people living in Jerusalem (Lk 13:4–5). Though nothing is known of this tower, it seems reasonable to conclude that it was situated in Jerusalem. Perhaps it can be identified with the great tower built by Nehemiah on the wall of Ophel (2 Chr 27:3; Neh 3:26–27) or with one of the towers built on the wall of Jerusalem near the pool of Siloam.
According to each of the 10 commentaries I searched through (and based on what I remember as well), there is no external corroboration of this incident. That said, the death of 18 people, while clearly significant to those involved, is not necessarily something that would be preserved in the historical record. Hundreds of people were crucified each year, and yet no extant list exists of that either.
The facts of the case, however, aren't really all that relevant to the point which Jesus was trying to make. It's a simple argument - God neither punishes or rewards based on man's merit. This is highly consistent with a whole of teaching on grace, God's soveirgnity, impassability, and the popular axiom 'There is nothing you can do to make God love you more, there is nothing you could have done to make him love you less.'
Consistent with the notion in Isaiah 55:8-9 (that God is just way above us), Isaiah 64:6 (our righteousness is as filthy rags), 1 Sam 16:7 (that God looks on the heart), is the simple idea that sometimes Bad Things Happen to Good People and sometimes good things happen to bad people.
Indeed, as Jesus himself says in Matthew 5, God causes the rain to fall on the just and the unjust alike. (Or, if he were using words today, he'd probably say, he makes the sun to shine on the just and the unjust alike). And, as John records:
1As he went along, he saw a man blind from birth. 2His disciples asked him, “Rabbi, who sinned, this man or his parents, that he was born blind?”
3“Neither this man nor his parents sinned,” said Jesus, “but this happened so that the works of God might be displayed in him. 4As long as it is day, we must do the works of him who sent me. Night is coming, when no one can work. 5While I am in the world, I am the light of the world.”
6After saying this, he spit on the ground, made some mud with the saliva, and put it on the man’s eyes. 7“Go,” he told him, “wash in the Pool of Siloam” (this word means “Sent”). So the man went and washed, and came home seeing.
The point is simple - not every bad thing is indicative of sin. But everybody is guilty of it, so everybody needs to repent.