www.patternlab.io
This website is extremely useful for creating patterns for web sites/interfaces. They advertise themselves as "PATTERN LAB: helps you and your team build thoughtful, pattern-driven user interfaces using atomic design principles." I really enjoy this site because it has a modern looking smooth user experience which is where web-design is headed these days. It helps me think/design an easy to use interface for all kinds of users. It helps me develop my design and thinking process.
www.fluidgrids.com
This website is a useful tool that shows web designers the different grid options for web layouts. It presents a 1-16 grid system and is useful in seeing the different ratios and use of the grid system. It's a useful visual.