When I went to QuakeCon last year, I saw a few people in costume, but not as much as you'd see at an anime con. I'll see if I can get the pics off my cell phone.
There was family dressed up as Mario characters and a Storm Trooper. The people selling Bawls also had costumes. I don't know what they wore for the first and last days, but they dressed as Mario and Luigi (their costumes weren't as good as the family's) on the second day and cowboys for the last day.I had some ideas for costumes. If I knew how to mod Nerf guns worth a shit, I'd mod a gun to be like the TF2 Heavy's Mini Gun and go as him to Quake Con. Another idea I had that I think would be easier would be to dress up as Marco or Tarma from Metal slug. I'd still need to mod a Nerf gun, though.
Here's an Instructable for a Headcrab zombie.
