I was rather curious myself.
M Tech comes up as a retail outlet selling other people's parts. I found a few vague references to branded mobo, but couldn't make anything of the website.
FIC is still around, going strong, doing a lot of OEM work. They are big enough to have their own Wikipedia page
Shuttle has it's own webpage and is apparently doing custom prebuilds with options not available to the standard use case as a side.
EPox closed in 2007 and reopened a short time later as SUPoX, they are also on Wikipedia
Soltek is the only one that appears to be actually gone gone. They still have products on Newegg as available, but business profiles indicate they went under.
Looks like of all of them FIC became a big player.