I'll just begin by foregoing my usual philosophical banter and get right to the tips.
Much of these vocal additives are relative to the song arrangement and creating different textures to distinguish the parts. Often it's a very subtle effect that can make it work. I mean where would T-Pain be without AutoTune?
One of my favorite vocal effects is creating a bastardized vocal with AutoTune.
I typically begin by tuning the vocal more naturally and meticulously with Melodyne.
I then process my lead vocal as usual. The vocal would ordinarily be just fine, but I choose to take it a step further.
I create a new mono auxiliary buss in ProTools. I will assign it to say Buss 1-2.
I then send the vocal pre-fader to that buss set at (0). This way I can run the effected track in parallel to the original.
Next I add AutoTune to that buss and set it to react pretty tamely. Next comes a SansAmp plug-in for some really cool distortion. Sometimes the distortion may be pretty bass and almost muffled, but for this effect it's OK. The words don't need to be intelligible. I then apply yet another AutoTune plug-in to respond radically from the harmonic distortion imparted from the SansAmp plug-in.
Now that the effect has been created you can automate it by riding a fader on your control surface or a mouse. I sometimes like to just tuck it slightly under the main lead vocal. When blended just right with the lead you can find a great tonal center. In this case you'll find that the lead may be more a composite of the two than just an additive effect.