How to create a huge choir with few voices?

As I have no budget to hire 200 singers, I would like to know what techniques and plugins I could use to, from a few recorded voices, create a multi-singing effect as if it were in a stadium?

http://www.quikquak.com/prod_crowdchamber.html
 
There may be some free ones too. go a googlin.

Lots of multy tracking and vary the distance to mics considerably. Stereo place the mics and leave them static -  literally vary which side you sing from. Vary your voice characteristics, pretend you're from another nationality and sing it in your language. Some tracks be a bit meek  - some tracks scream your head off. I wouldn't suggest you use plugs at all. It's the variation and missed timing that sells it.  My opinion only - but I've done it a few times.

You might want to demo MUnison from Meldaproduction. To me, it sounds kinda fake on voices, but if the synthesized voices were set back in the mix from the real voices, it might work. 

Just coincidentally ran across this video today and it made me think of this thread.
 
Yeh, it's just an ad for Waves, isn't a direct answer to the original post, nor does it reveal any arcane secrets that everybody here doesn't already know.
 
But it's still pretty cool to see this demonstrated by someone who's well known for that wall-of-sound effect. If you've ever listened to any of Devin Townsend's more epic material, you already know that he's a master of the technique.
 
The key lesson here is that even though he's hawking effects for Waves, he doesn't synthesize any of the vocal parts with doublers or the like. Instead, he takes the time to actually record gobs of individual vocal tracks (all him, which I found surprising) and mix them into to a grand mush.
 

It's always good to get someone else's voice in there, even if they're not a good singer. You're trying to assemble a rich mix of overtones and formants, which is next to impossible when it's the same voice doing every part.
 
When my granddaughters were 11 and 12, I recorded them singing some oohs. They were shy so it took a lot of prodding to convince them to do it. But after tuning and treating the tracks, they sounded like a choir of angels and were pleased with themselves. I had them each sing the same notes of the triad, six tracks in total, each treated and panned differently. It's similar to the technique used on Bohemian Rhapsody, which had three takes of three singers, who swapped each others' parts for each take.
 
I'd read of the same technique being used to create a violin section with one violinist. They set up four chairs, which he rotated between while overdubbing the parts. Why not just pan each part? Because when it's acoustical instruments, it's not just about panning; the room sound is different from one position to the next.
 
Which makes me wonder if the same trick might not work for vocals. Not necessarily moving around the room, which wouldn't help since you're probably recording in some kind of dead space. But you could vary the distance and angle from the microphone for each pass, or even change out the microphone, thus getting a unique tone for each take.

I use waves doubler and reverb. Sometimes I use melodyne but doubler does a great job on cloned tracks