The 1000 Problems site is an excellent collection of resources organised into collections on Number, Algebra, Shape and Space and Statistics. Within each category problems are organised by age and key words and a clear description are given for each problem. Extension questions are available for each category. For each problem a file can be downloaded which includes solutions.
1000 Problems – Number, Extension
The other 1406 Problems are the results of a search on The World Mathematics Championships on Nrich. A large collection of problems can be found on Nrich and many more sample problems from selected rounds provide a rich source of problems.
Looking at the Mathematician sample problems for example we see a collection of mostly Junior (Junior level is aimed for those in Middle School/lower High School (in Grade 9 and below)) resources including mixed question sets and also some by topic.