Smallest possible Ajax implementation (120 characters)

ajax120.js

function a(u,c){var x=new XMLHttpRequest;x.open('GET',u);x.onreadystatechange=function(){3<x.readyState&&c(x)};x.send()}

116 characters:

function a(u,c){with(new XMLHttpRequest){open('GET',u);onreadystatechange=function(){3<readyState&&c(this)};send()}}

(Thanks to @fearphage for reminding me of with!)

What do you even pass as the "c" parameter, though?

The c stands for “callback”, so you pass in a function to get run (and passed the XMLHttpRequest object) when the request returns.

Using commas instead of semicolons means you can get away without the {}, so you could shave off a couple more characters :p