It's called AutoTune.

Autotune

This effect is used in a lot of songs these days, if you are producing hip-hop you'd better get it asap. I always refer to it as "the T-Pain effect" since that guy has pretty much used it to death. I think you can still get it for free (as a demo) from the Antares website.
The effect was designed to subtly correct a singers pitch, but somewhere along the line people started using it at extreme settings to create the vocoder sound you are looking for (the first time I heard it was the Cher song "If You Believe in Life after Love.")
You'll probably need to tell Autotune what notes you want it to "fix" the notes to, in a key that works for the rest of the song. If that makes you say, "Huh?" then just experiment until it sounds good.
This link should get you started:
http://www.associatedcontent.com/article/917213/how_to_use_auto_tune_to_get_the_tpain.html

FYI I was just looking at http://www.antarestech.com, & they still have the free demo version. The deal is, the plug-in stops working after 10 days, so you could probably do your project with it for free.
Also, they have a new version called AutoTune EFX. It looks like its a more simplified version, that helps you get that "T-Pain effect" without having to do as much programming.

Thanks for the help guys!
Song is sounding good :)
This has helped alot