Recommender systems provide support in accessing useful or interesting content. In the recommendation process individual characteristics of the user are taken into account in order to provide personalization. Recommendation techniques are also used in the domain of e-learning, but in the most of existing systems set of items that is recommended is limited to set content to learning materials and courses in general. In the e-learning which includes collaborative learning activities (e-tivities) different aspects can be adapted to students' characteristics. Thus, recommendations can also refer to the appropriate collaborator or tool for e-tivity, advice that will support students in its implementation, or e-tivity as a whole. The dissertation presents own model of the recommender system that enables personalization within a structured sequence of collaborative e-tivities realized using Web 2.0 tools. The system contains the following components: the activity model, the subsystem for user modeling and the subsystem for determining recommendations. The activity model represents course activities and items that can be recommended. Those are: optional e-tivities, collaborators, Web 2.0 tools and advice. The subsystem for user modeling represent student's and group's characteristics that are used for personalization, and includes the original procedure for activity level estimation for students and groups. The activity level represents quantity and continuity of student's (group's) contributions in e-tivity relatively to other's contributions. The data is calculated based on certainty factors theory and, together with other user characteristics, used in the recommendation process. Recommendation process is carried out within the subsystem for determining the recommendations which contains pedagogical rules. Teachers modify existing rules in order to define the recommendation criteria. The prototype of the system was built and called ELARS – E-learning activities recommend system. The proposed methods, algorithms and pedagogical rules were implemented within ELARS system and verified in the actual environment. The results of the evaluation indicated that the system is effective in the sense that the students who use it achieve better results on e-courses. Additionally, the results of a questionnaire showed that the students find the system useful for e-tivities, that they are satisfied with its interface and received recommendations.