Jakob Nielsen's 10 general principles for interaction design. They are called "heuristics" because they are broad rules of thumb and not specific usability guidelines.
Creating Empathy Maps is useful for synthesizing research data in order to better understand the users. Empathy Maps are great for creating Personas, identifying users with different mindsets.
Affinity diagramming has long been used in business to organize large sets of ideas into clusters. In UX, the method is used to organize research findings or to sort design ideas in ideation workshops.
