Challenging online puzzles?

[MatsukenDG]

How do I choose from the available sites a resource with free puzzles that offers both simple and very complex puzzles for different levels of training?

[Zavylon]

My skill level fluctuates depending on time and patience. Early platforms offered limited challenge ranges, which quickly became boring. Now I look for resources that provide both simple and complex puzzles. Finding a balanced range within a system such as online jigsaw puzzles reassured me that growth and variety can coexist within one cohesive platform.