From the Perian team:
We began the Perian project over 6 years ago. We wanted to simplify viewing your content. Our team has attained that goal and with that in mind, Perian will be retired soon. Our stewardship has been a blast but it’s time for all of us to move on.
For those who don’t know, Perian is a plugin which helps Quicktime on your Mac to play video formats that it natively won’t. I use it alongside Flip4Mac, which does the same but with Microsoft specific formats. The guys from Perian are suggesting that after their final release (which will be supported for 90 days after release), you should download Niceplayer.