The RE2 and IR have composite and S-Video outputs built in to the graphics boards. These ouputs show a 646 x 486 (768 x 576 in PAL) region of the frame buffer. On the RE2, the area that is shown can be controlled using the command /usr/sbin/vout; on the IR, it's controlled by ircombine.
The Onyx (RE2 or IR) also supports the Sirius video option, a separate board (plus breakout box), which provide several composite and component video input and output options. /usr/sbin/videoout controls the live video output of the Sirius.
On the Onyx2/IR, the DIVO board provides digital video input & output, similar to the Sirius.
On the O2, the O2 video card provides composite video input & output.
It also works with /usr/sbin/videoout.