These events are proposed and organized by SIGGRAPH attendees, not the conference organizers (who simply approve them and provide rooms). They range from large, elaborate presentations to small meetings. The list of Birds of a Feather (BoF) events (with dates, times, and locations) is available here.
The OpenGL BoF is one of the largest and longest-running. Each year, it is the premier event to hear about the latest developments in OpenGL. This year it is joined (actually, preceded, by one day) by the OpenCL BoF, which discusses this new API for GPU-based general computations.
Although most render farms are used for film production, many game developers also have render farms which they use for lighting and visibility precomputations. For this reason, some game developers may wish to attend the Renderfarming, Job Queueing, and Distributed Rendering Performance event.
The Computer Graphics for Simulation BoF and the Dynamic Simulation Birds of a Feather also seem relevant for game developers, although they are not specifically concerned with rendering.
The Interactive Ray Tracing BoF has already been mentioned by Eric, and is of interest to many readers of this blog.
One of the uses of real-time graphics with which I have very little familiarity is visualization of molecules, which has its own BoF event (the Molecular Graphics BoF).
Besides BoFs for people interested in specific graphics topics or products, there are also BoF events for particular groups within the graphics community, such as the Women in Animation BoF and SIGGIG: Gays in Graphics. One of the most interesting of these is the Computer Graphics Pioneers Reception, which is intended for people who have been contributing to computer graphics for at least 20 years (if you fit this description and are interested in being a registered Computer Graphics Pioneer, the membership details are here).
Other BoFs are intended for people from particular regions or who went to certain schools, like the Taipei ACM SIGGRAPH Reunion, the ACCAD/OSU Alumni Gathering, the Tokyo ACM SIGGRAPH Chapter Party, the UNC SIGGRAPH Alumni Reception, Reuniao dos Brasileiros – Brazilian BoF, Purdue University Reunion, RIT Alumni Reception, and Reunion de los Mexicanos – Mexican BoF.
Finally, many regular SIGGRAPH attendees like to go to the Sake Barrel Opening Party BoF. I haven’t attended one yet, but perhaps I will this year.