Nik Collection is a set of photo editing tools, consisting of Analog Efex Pro, Color Efex Pro, Silver Efex Pro, Viveza, HDR Efex Pro, Sharpener Pro, and Dfine.
Initially, the collection was offered for $500 and later it was dropped to $150 when Google acquired the company. Now Google has decided to provide these photography collections entirely free of cost.
If you have purchased the collection in 2016, you will be eligible for a full refund.
Here is the announcement from Google.
You can download the entire Nik collection here.