seriously, if you go back to the '07 postseason, maybe view the Super Bowl, you'll see Brady using it when he's not getting crushed by Justin Tuck. Who was first, I don't know. I just heard Brady using it and never noticed Manning used it to until this weekend. I can see how when you call a lot of plays at the line, either audibling or going no huddle, you need to signal to your players "snap count is coming." Manning went to that style before Brady, but Jim Kelly probably first used something like it.
[as for the steaming bit, jk. Brady doesn't steam at anything other than receivers who don't make the adjustment he thinks is right]