Contributions to open source software provide evidence about developers' expertise and roles. Moreover, information about developers' activity can assist in identifying their competencies in software technologies. Existing project-centric approaches benefit the needs of expert identification within a project, but provide a limited view of developers' expertise. In this paper, we identify contributors' expertise and roles by considering their contribution history across multiple projects according to different technologies. Firstly, we identify terms related to software technologies and employ information from GitHub to extract contributors' activity on specific technologies. Secondly, we present four contributor roles: developer, technical leader, bug fixer and bug contributor. We study the contribution history of 2,973 users of GitHub and reveal trends in contributions in open source software that can be exploited by employers for identifying experts or by practitioners for showcasing their expertise in various technologies.