The main motivation for organizations and individuals to collaborate is to enable knowledge and resource sharing in order to effectively fulfill a joint business opportunity. This paper focuses on virtual organizations (VO) and virtual teams (VT), whose strengths lie in the range of competencies of their members, offered jointly through collaboration. One of the difficulties in VO and VT creation is partner selection using partners' mutual trust as one of the selection criteria. This paper provides an analysis of trust relationships based on the principal-agent theory, and proposes an approach to hierarchical multi-attribute decision support based trust estimation applied to a network of collaborating organizations (VO) and a network of collaborating individuals (VT). The paper presents two case studies, one using a questionnaire-based approach and another using automated reputation and collaboration estimation from data gathered by Web crawling.