The most popular ones

• Atomic CSS - One Rule for One Styling
• BEM - component-based approach, any component level class is a block, any its part is an element, the state of both block or element could be described with modifier
• ITCSS - Inverted Triangle CSS divides CSS structure into sections going up-down from more general to more specific
• SMACSS - separate our classes into groups Base rules, Layout rules, Modules, State rules, Theme rules, name and structure them accordingly
• OOCSS - introduces wide code reuse with extracting repeating CSS patterns as default classes

They can be used in combination together as well as independently. Take the best of each approach.

• 5 Methodologies for Architecting CSS
• CSS Methodologies (stateofcss)