You can buy replacement focus screens for Nikon and Canon bodies with the split-image in various configurations.

The one I was already familiar with is Katz Eye:
http://www.katzeyeoptics.com/

But there are others:

http://www.focusingscreen.com/
http://brightscreen.com/enter.html (their HTML web page looks like complete shit, I shudder at the thought of seeing their Flash version)
http://haodascreen.com/othercameras.aspx

The benefit of these is that you don't have to walk around like a pretentious Leica user.