Back to Joe's question:

Slapback is probably one of the most distinct effects that was 'signature' to a whole era. Silverback's description was great.

Phil Collin's had what I would call signature effect on his solo vocal stuff after "In The Air Tonight" went huge. Not exactly sure what all was involved, but my guess is there was some doubling, delay, etc. involved to thicken his rather thin sounding voice (my opinion).

Lots of folks have used doubling for their 'signature' sound. Another, although less famous person, is Elliott Smith's vox - almost always doubled.

Nick Drake is another where it sounds to me like lots of doubled vocals.

Karen Carpenter is another if I'm not mistaken.

Enya got her signature sound in Orinoco Flow by using 'doubling' to the extreme; supposedly tens of takes were used, stacked on each other. Online article exists on that - just a second I'll find it. Ah, yes, there it is: http://enyabookofdays.com/articles/wm-tv04.htm

Look in there and Enya claims there were over 100 tracks of vocals that were layered for Orinoco Flow, probably her signature song.

Anyways, I would start by experimenting with layering multiple takes. Sorry, but I'm of the opinion that this is not a live effect or a 'push the preset' type of effect. There are attempts to do this in DSP, but to get the real deal, give the real deal a try. I'm always doing multiple takes of my vocals anyways. I just use those where I've done a decent job of hitting the timing between 2 or 3 takes and try stacking them, with some slight panning differences between them.

Then you have the over-used 'telephone EQ/distortion' effect on vocals - not sure it's a signature sound for any one artist.

-Scott