The first Super Smash Flash was originally submitted to Newgrounds almost 8 years ago, in 2006. You can play its sequel right now, though it is still in development—in fact, it's getting a huge update very soon.
Essentially, the game is (currently) a stripped down version of Super Smash Bros. Brawl with different stages and characters. It's the basic Smash Bros.experience, with a robust Versus Mode and some Event stages, but there's a few key differences: it's free, it's on PC, it uses pixel art, and the production quality varies a bit in places—this can mostly be felt in the music bitrates.
Still, it's a pretty damn entertaining little distraction, and gameplay-wise, to me, it feels as responsive as its console brothers. And with the upcoming update, it's getting even better, with the addition of special game modifiers like Slow, Mini or Turbo, an online mode, and three new characters: Chibi Robo, Marth, and Zero Suit Samus (with Isaac from Golden Sun coming in a future update). The vid below has more details:
You can give Super Smash Flash 2 v0.9a a try over here, at its official site (you can play it in your browser or download it). The big update is scheduled to arrive tomorrow.
Super Smash Flash 2 v0.9b [McLeodGaming@YouTube, via DSOGaming]