PSprices.com is my favorite site to keep up with PS Plus sales (and all other game sales). I like its simple look, and there are a lot of useful features, like ...
sort by a number of filters, including New deals, lowest-ever price, $ off, % off, etc.
see a chart for each game that shows the history of its price point, so you can calculate when it may go on sale again.
sign up for emails that notify you when a game goes on sale.
see other platforms sales (including PS 3 and Vita, which I personally still use sometimes).
a slideshow of images and a video review of the fame
etc.
I check this site ever other day or so. They even tell you Epic Games (PC) sales in case you have a PC as well.