Is there a document somewhere describing each of mplayer's video output drivers and why'd you'd want to pick that one for a given circumstance (or why it exists) Would someone be willing to write that out here?
The video output drivers compiled into your version of mplayer can be viewed by running
mplayer -vo help
As to which you should pick when, some of that will be obvious from the help output (for those that target specific video cards[ mga, s3fb, etc], or output formats [aa, png, etc]). Some are obsolete (I don't think VIDIX went anywhere, or GGI). The others, I cannot help you with. I use
xv when I can and fall back to
x11. The others are somewhat specialised so unless I find a reason to use one, I wont.