Standard Junction objects available in Salesforce are

1. OpportunityLineItem

Junction object between Opportunity and Product objects.

2. PriceBookEntry

Junction object between Product and Price Book objects.

3. CaseArticle

Junction object between Case and Knowledge Article objects.

4. CaseSolution

Junction object between Case and Solution objects.

5. OpportunityContactRole

Junction object between Opportunity and Contact objects.

6. AccountContactRole

Junction object between Account and Contact objects.

7. CaseContactRole

Junction object between Case and Contact objects.