You can do a lot of things to a voice but I'll try to put all devices in order of importance.
1. Mic (+plop screen) (I'm not an expert in this comparment and it's very important for vocals, seek some help with persons who know.)
2. Reverb: smooths out the voice, don't overdo it.
3. Compressor: Some compression and a noise gate will make your recordings sound better.
4.De-esser, or Parametric EQ to get rif of the sssssssssssssssszzzzzzzzzz.
Some other, but not necesairry items: Vocoders (for those Robot type things and sounds à la underworld), Pitch shifters (those high voices and looow voices (daft punk ' teachers')), Auto-tune (to sound in tune if ya can't sing in tune (anything by the 'spicegirls')), vocal distortion, delays, flangers, ....etc...