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 collection entirely free of cost.
If you have purchased the collection on 2016 , you will be eligible for a full refund.
Here is the announcement from Google.
You can download the entire Nik collection here.