How can a PhD student enlarge his academic collaboration network? I am a new doctoral candidate of information and computer science and about to initiate my research, and typically collaborate with my supervisor and other group members. But seldom will a researcher limit his collaborators to those within the department he's in; he has to enlarge his network. I believe energetically participating in academic activities/events surely brings benefit. As a student, my quick thoughts are, for example, applying for internships at some research institutes, summoning team members for contests, or taking part in open source projects. Can you kindly share other experiences?
Of course, you’ll get there by engaging in a lot of activities where you will meet new people and work with people who will learn to know you. I recommend choosing them according to a few criteria:


Activities of a seemingly technical (or practical) nature. As a PhD student, you probably don't want to be on boring committees, board-style meetings, interdepartmental seminars. You will meet people, but not in a way that fosters collaboration. (However, you may want to attend these events for other purposes. Meeting key people involved in hiring decisions at a given institution is one such purpose that comes to mind.)
Informal settings. This favors meeting new people and getting to know them much better. Favor small meetings over big ones. Aim for a few persons you want to meet. Identify people you would like to approach (at a big event), then check what events they attend.
Manage to invite people for talks at your institution. Okay, this one might be a bit difficult for a PhD student in some places, but if you can manage to get someone invited for a day or two, get him to give a talk and discuss your research and his, it will be worth it.
Do not shy away from “learning” events: tutorials, “hands-on with XXX library” type of things.


All in all, I think the kind of events you want to engage in are:


Open source projects: you listed that one already
Workshops close to your research topic: these typically involve few people and long discussions. Many of my strongest collaborations (and a few friendships) grew out of workshops.
Tutorials organized on topics related, but not too close, to your research: you will learn stuff, meet people in a relaxed setting. Don't be shy of going to workshops where you already know some of the stuff, if only to put it in practice and discuss with the best experts. Also, be ready to present your own work and identify convergences with lecturers and other participants.