Most of the good stuffs can be achieved inside Fl studio using its internal effects , like the parametric EQ 2 , reverb 2 , love philter, , the vocoder ,the phaser and the chorus ...
But you may want other cool free vst like for the reverb :
http://www.ariescode.com/index.html?htt ... sverb.html
http://www.dasample.com/index.php?show=glaceverb
Ambience :
http://magnus.smartelectronix.com/
Multi effects :
Luxonix LFX :
http://www.luxonix.com/home/en/products.html?id=lfx1310
Choralzoide :
http://perso.orange.fr/numerikart/products.htm
Of course on vocals you should use these effects with parsimony , depending of course of which goal you want you to reach.
Last but not least experiment is the key , when you have found some good chains of effect , remember to save them as *.fst files in order to use them later ..
Downloading the Free Blue cat's Bundle is also a good way to expand your plugin arsenal .