The Graduate and Professional Student Council (abbreviated GPSC and pronounced like "gypsy") maintains the above list of extremely helpful information. GPSC holds many social events during the semester for students from all graduate departments as well as professional school students (including medical, business and law). GPSC represents graduate and professional student perspectives to the faculty and helps handle issues like the prices of health insurance and parking. GPSC also oversees the distribution of graduate student tickets to men's basketball games, running a weekend long line known as "campout". The GSO has a representative on behalf of the physics department in GPSC.
The graduate school maintains the above list of resources for grad students. They hold social functions (usually pizza or barbecue and music) a few times a semester where you can meet graduate students from outside the physics department.
The APS FGSA provides a national forum for physics graduate students. They have resources targeted specifically at the needs of such students and conduct surveys of physics graduate students to determine their needs. The FGSA has several meetings and workshops annually that address both research and graduate life/caeer topics.