Looking for a bit of nostalgia, I'm trying to think back to some older Flash games which were any good.
Overall quality in Flash games has improved over the years but even years ago there were some decent ones.
It can be any old enough Flash game, preferably from Newgrounds' early years or even predating Newgrounds, which is good enough it'd still be worth playing.
An example of such a game would be Pol http://www.newgrounds.com/portal/view/37611