Rich Questions – my post on resources which give questions requiring higher order thinking skills.
Nrich has extensive resources. Jennifer Piggot has written an article on Integrating Rich Tasks, this includes a complete series of professional development resources designed to support teachers to integrate rich tasks into classroom practice. Their curriculum mapping documents provide a helpful way for teachers to find resources. Also note the Nrich Packages, these include a set of tasks on working systematically and the Nrich poster collection (scroll down to the bottom of the poster collection page to download the collection as a PowerPoint presentation).
The Nuffield AMP Investigations are designed to teach and assess key mathematical processes. Each task has detailed teacher notes.
New Bowland shorter assessment tasks include worksheets, notes and an optional presentation in three different formats, Word, pdf or PowerPoint.
From AQA comes an outstanding resource, GCSE Mathematics: 90 maths problem-solving questions. indices by topic and by strategy are included.