Google has announced a new beta of its Chrome browser for OS X which comes with support for extensions. It's been available on the highly-experimental developer channel for a little while, but this is the first release for the functionality in beta.
The beta also includes other features that Windows users have had for a while, including bookmark syncing - which requires a Google account and for you to sign in. There's more than 2200 extensions already available for Chrome, though that doesn't touch the whopping 161,000,000 in use for Firefox.
If you're already using the Chrome beta channel, then you should be automatically updated to the latest version within the next day or so - it's version 5.0.307 that you're looking for. If you're on the normal channel, or you don't use Chrome at all, then you can install the beta from scratch.