From their announcement on Google+:

Photo enthusiasts all over the world use the Nik Collection to get the best out of their images every day. As we continue to focus our long-term investments in building incredible photo editing tools for mobile, including Google Photos and Snapseed, we’ve decided to make the Nik Collection desktop suite available for free, so that now anyone can use it.

The seven plugins — Analog Efex Pro, Color Efex Pro, Silver Efex Pro, Viveza, HDR Efex Pro, Sharpener Pro and Dfine — are now free to download from the Nik Collection website. They work with Adobe Photoshop, Elements, Lightroom, and Apple Aperture.