Celemony Melodyne and Antares Autotune when used with skill can pull a whole bunch of off-key notes as straight as you like.

Melodyne can even isolate the harmonic components and adjust them separately, which means it can fix a guitar with one string out of tune with the others. I guess the result doesn't sound too great when you have to do that, but it might just be better that an out of tune instrument. On single notes it works very well. I've seen it used - the note pitches show up on a screen and fixing problems is about as difficult as fixing spelling errors in a word processor with a spell checker...

The effect is deliberately overdone on pop vocals everywhere, but when used carefully to fix minor pitch problems it often isn't noticeable.