Google Open Source - All of Googles open source projects under a single umbrella
Placemat - Pretty nice placeholder images.
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.
PlaceKitten - Dead-easy placeholder images. Of kittens.
Open Source @IFTTT - A collection of IFTTT OSS projects.
placeholder.pics - Lightest way to include placeholder images in your design