Primeshooter, from 1729.com. You shoot numbers with their prime factors; when you hit a number with one of its prime factors (2, 3, 5, 7, 11, 13, 17, or 19) it is divided by that number. Once you reduce a number to a prime, you can hit it with a "prime" bullet that makes it go away; numbers also go away if you successfully get them down to 1. You lose a life if a number hits you; if a number reaches the bottom of the screen without getting down to 1 it reappears at the top. Sometimes (I don't know when) it appears in a bright red box, in which case you'll lose a life if it reaches the bottom. Sometimes you get an extra life.
I got a score of 834 (= 2 × 3 × 139, although that's irrelevant), which means I successfully shot at 834 prime numbers. I don't recommend trying to beat me, because even if you succeed you will lose about thirty-five minutes of your real life.
The rest of the web page looks kind of fun, too.
An exercise for the reader: why is it fitting that this is on 1729.com?