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Extensions were already available to Windows users, and to folks who use Mozilla Firefox, they're foundational in the browser experience. So you can imagine how anticipated their arrival is.
Extensions were technically already supported under Chromium's hood, but you couldn't install them from the gallery. While there were some easy workarounds for that, now you don't even need those; just click install and you're rolling.
Take a step over to Google's early access release channels page to download the developer version of GoogleChrome.dmg if you're interested. There aren't many extensions available yet — certainly not as many as you'll get with Firefox — but there are enough to make it worth a try.