Blu-tack

Blu-tack is a reusable putty[?]-like pressure-sensitive adhesive often used for attaching posters to walls. The original version was blue, but a white version is now available. It is an oil-based synthetic rubber polymer.

Blu-tack appears to be a product of, and presumably a trademark of, Australian company Bostik Findley[?], a subsidiary of TotalFinaElf[?].

Similar products from other manufacturers include "Buddies" (pink), and "Pritt-Tack".