The open source perspective offers an interesting insight about cloud computing technologies: in one hand, cloud systems belong to the category of the Ultra-Large-Scale (ULS) systems, i.e. very complex systems where conventional approach for the technological development does not work. For such as systems, Free Libre Open Source Software (FLOSS) licensing attracts innovation from the developers’ communities, reduces the risks of technology adoption and fosters the interoperability between systems and the creation of open standards. In the other hand, the current systems are far from achieving interoperability; even the FLOSS´s principles remain pending for many components in the architecture of the main cloud solutions, and for these reasons many FLOSS evangelists do not recommend using them. As a balance between the obvious drawbacks and benefits, recently a new strategy has appeared: Free/Open Services. However, it seems difficult to find short term solutions. This chapter illustrates both ideas, highlighting the pros and cons of these technologies, including a reference of main “open cloud” groups and open source technologies for the cloud. The rest of the book will include additional and deeper descriptions of some of the most interesting open cloud technologies.