Content Marketing Stack - A curated directory of content marketing resources
Placemat - Pretty nice placeholder images.
Top Instagram Influencers Ranking - Curated directory of Instagram bloggers by country&category
PlaceKitten - Dead-easy placeholder images. Of kittens.
Startup Stash - A curated directory of 400 resources & tools for startups
placeholder.pics - Lightest way to include placeholder images in your design