I'm envisioning a graphical app that displays a file. Like a pager, the up and down arrows move through the file. But the left and right arrows move through time. As each successive change to the file is displayed, the committer's name appears in a column to the left of the lines changed in that commit. Hover the mouse over it to see the commit message. Names of old committers will fade out as time advances, but still be visible for a while. (A menu option will disable the fade out entirely.)
A nice bonus feature would be to allow opening multiple windows, with multiple files from the same repo. Moving back and forward in time would affect them all at once.
A nice, but getting harder feature would be to have a horizontal timeline at the bottom, including branches, so you could click on a specific branch to visit it. (Without this, when passing a fork or merge point, it would have to choose a branch heuristically?)
A tricky subtle feature would be to attempt to keep the current code block centered in the display as lines are added/removed from the file, adjusting scroll bar position to compensate.
There seems to be a gannotate for bzr, that may do something like this.
Offline so I can't try it.
Google-and-caffine-fed update: bzr gannotate is closest to what I envisoned, though without a few of the bonuses (fade-out, smart scrolling, multiple files). qgit's "tree view" includes the same functionality, but the interface isn't as nice.