Placemat - Pretty nice placeholder images.
OfficeMixer - Automated office bookings
PlaceKitten - Dead-easy placeholder images. Of kittens.
RobinPowered - Robin coordinates meeting spaces, people, and things in your office. Complete with analytics and insights that uncover usage and help optimize space.
placeholder.pics - Lightest way to include placeholder images in your design
Officely - The only desk booking tool that lives in Slack