The Nik Collection is a number of filters and effects for digital photos: Analog Efex Pro, Color Efex Pro, Silver Efex Pro, Viveza, HDR Efex Pro, Sharpener Pro, and Dfine. These are all free to download but will not be maintained further on. It was developed and sold by Nik Software, etablished in 1995, which was bought by Google in 2012. google continued to sell the Nik Collection until the start of 2016, where it announced that the software would from now on be for free, along with announcing the end of Picasa. However, Google didn’t state clearly if it intended to continue the development, but they refunded the customers having bought the product in 2016, and the product had not been updated for a long while, indicating that, in fact, the product may be discontinued.
There are other similar and further updated products on the marked, especially the range of Franzis Projects software, which has very powerful features, a broader compatibility, and can work as stand-alone programs as well.
For more details on the Google products, have a look at the Google Nik Collection website from where the individual tools can also be downloaded.