No magic bullets that I know of. The voice, the mic, and the preamp are going to determine 90% of your sound. I always suggest front-loading the recording chain in that way. There's no way to fix a bad vocal track with plugs.
That being said, there are a few (free) plugins that I use commonly on my vocals:
Compressors:
Digital Fishphones Blockfish (has an 'opto' mode and an 'air' button to make you feel more special)
Kjaerhus Classic Compressor - a solid all-around compressor plug
Reverb:
Kjaerhus Classic Reverb
SIR Convolution Reverb
GlaceVerb
Delay:
Kjaerhus Classic Delay
De-Esser:
Digital Fishphones Spitfish
Tube Emulators (I don't usually use these, but they can be helpful for certain sounds):
Steinberg's 'Da Tube'
Ruby Tube
Those are generally the only plugin effects I put on vocal tracks and I tend to use them very sparingly.