Brilliant

This is a fantastic site with very challenging problems.  I would recommend this for people studying for standardized tests. The questions in a category start out very easy and then you progress to very difficult problems.  This is a problems solvers type of site

This site covers subjects like:
Logic
Word problems
Physics problems
Algebra problems
Many more types of problems

This site asks multiple choice questions that are very much like questions that you see on standardized tests.  This is a great site to practice for these tests. You also move to another level each time you master a bank of tests.  The questions get very challenging and are great test taking practice.  To join, you do need to provide an email and you will get quite a few emails challenging you to solve a problem.  The tests stop at a lower level and you have to pay a subscription to be able to do the more challenging problems.  Without paying, there are still many challenging problems.

This site is appropriate for:

Students taking standardized tests.
Students from grade school through graduate school
People who are life long learners
People who like challenges