Nrich has extensive resources including articles discussing the nature of rich tasks.
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 which includes numerous problems. Click the resources tab then look for the Additional Maths – Problem Solving Questions & Teachers’ Guide.