Many online design tools are available and widely used by electronics designers. These tools help engineers to explore, learn, design and build their projects from scratch. They have some cool GUI which has input/output indicators, push buttons, etc. that simulate a real system. Out of those famous tools is the WEBENCH Designer from National.
Design assistant tool that takes step by step till you generate Bill of Material (BOM).
I like these tools because they guide you to the right design path.
Click here to see WEBENCH