There is a web site that I visit daily. Okay, well there are a number of them, but I want to tell you about this one specifically. It is FreeKibble.com. The default is for dogs, but there is a cat portion as well.
The way it works is thus: if you go daily, and answer the trivia questions, they donate 10 free pieces of Halo pet food to shelter dogs and cats; here's the analysis of it on Dog Food Advisor. It isn't on Dog Food Analysis, that I could tell, but their search function can be wonky. It doesn't matter if you get the question right or wrong, you just have to go. 10 pieces of kibble isn't a lot, but if you count it out and look at it, it's better than nothing. Since Free Kibble started, many shelter meals have been provided through these donations.
In addition to that, they've just released their first book, Mimi and Maty to the Rescue! You can buy it on Amazon.com (linked below) and every purchase of the book donates FIVE BOWLS of food! Not pieces of kibble, bowls! Also, for the first 100 copies sold, the Freekibble Foundation will donate 100 books to schools to "inspire young readers to help animals!"(link to the promo page on the Free Kibble site here). It looks like a cute book, and is certainly a great message and a good cause!