Focus Follows Mouse or Sloppy Focus on Mac OS X
The standard Terminal application included in Mac OS X will do focus-follows-mouse (within that application only, and no auto-raise) if you run this command from a shell and then restart the Terminal app:
% defaults write com.apple.Terminal FocusFollowsMouse -boolean YES
The fundamental problem with sloppy focus on the Mac is that the menu bar is always associated with the currently focused application; if you had sloppy focus, accessing the menu bar for a specific application would be supremely difficult.
Having said that, Zooom/2 does what you want, in addition to providing equivalents to the open-source window manager features for ctrl+click moving & resizing of windows.
Steve Yegge over at his blog claims that focus follow mouse with no autoraise isn't going to happen on OS X anytime soon:
http://steve-yegge.blogspot.com/2008/04/settling-osx-focus-follows-mouse-debate.html
Since ALL OS X utilities out there that promise "focus follows mouse" do autoraise, I think Steve Yegge is sadly right.