Silver Efex from the Nik Collection has long been touted by many as one of the best mono photo processing tools available. Bundled with a number of other plug-ins (colour, sharpening etc.) it used to be quite expensive at $500.

Then Google bought the lot and soon after started charging only $149 for the full set.

Google's now gone a step further and the whole collection is available for free from 

https://www.google.com/nikcollection/

Many are suggesting that Google only wanted Snapspeed (created by Nik and using the Nik Collection tools as its foundation) and that Silver Efex et al have effectively been killed off as there will be no further development. Of course this doesn't take away from the tools in their present form.

I've tried the free Silver Efex and it has an interesting and appealing interface, but I think I'll stick to my current workflow or Adobe Camera Raw for the majority of my processing and Photoshop for the things ACR can't do.

Incidentally, Snapseed is my photo app of choice on my iPhone.