i think with vocals two effects (if you actually CAN call them effects) which are almost always used are compression (i use waves. think it's brilliant) and eq (several plug-ins).
with these it's possible to make the vocals sit properly in the mix.
then there's reverb of course (freeverb is ok, i use waves). not only to make the vocals bigger (I never achieve that) but to give them the bit of stereo back that you lost in the tracking stage. also it sounds strange to have a considerably different amount of reverb on the instruments than on the vocals (if not wanted as an obvious effect).
closely related to reverb is delay. almost any plugin will do as long as you can roll off the highs. sometimes you can get nice results using only delay instead of a reverb. sometimes a combination is what you need.
a slap-echo (short but still noticeable as delay, mostly mono and centered), a wide spread stereo-delay (perhaps 30ms, spread almost fully left and right) or a single 8th- or 4th-note delay (almost inaudible) is what I use most. depending on the song.
sometimes distortion can add a certain something. i once mixed vocals using only compression, eq, distortion (mda plug ins, free) and a delay. they sat perfectly in their distorted guitar environment - i was amazed. but with a cleaner background the distortion on the vocals was kind of bothering, but turning it of made the vocals sit less well. strange things happen.
exciters can be a nice thing too. but one tends to overuse them. compared to a vocal track using an exciter, the "clean" track always sounds - well - less exciting, but nevertheless I think excited tracks sound a bit artificial.
chorus and flanger (i like blueline here) are also handy to polish up your vocals. normally I only use them as an obvious effect for "psychedelic" stuff (like hendrix covers or whatever). using only a bit can make the vocals sound more professional... or just overprocessed

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the thing is, putting these effects on a vocal track will always make them sound "better" compared to the original track. it will also make them sound more unnatural though.
i always go for "less is more". you don't have to use fancy effects if it already sounds good.