English numeral calculator
First of all, this is totally cheaty, and not complete to the specs.

requires the --disable-web-security flag on chrome, +22   

Javascript 509 + 22 = 531

x=new XMLHttpRequest;y=Object.keys(x);b=alert;q="querySelectorAll";s="send";x[y[3]]="document";x.open("GET","http://www.wolframalpha.com/input/?i="+escape(prompt()));x[y[10]]=function(c){4===x.readyState&amp;&amp;(w=[].filter.call(x.response[q](".pod h2"),function(a){return"ame:"==a.innerText.slice(-4)})[0].parentElement,(k=w[q]("a")[0])&amp;&amp;"Words only"==k.innerText?(x.open("GET",k.href),x.send()):alert(JSON.parse([].pop.call(x[y[2]][q]("script")).innerHTML.match(/d_0.00\.push\((.+?)\)/)[1]).stringified))};x[s]()


The first off the spec is as well the example output
The input two plus two minus five times five divided by negative zero point five outputs



Any other case should get handled fine (now), this is yet pretty ungolfed as is, I just wanted it to get fixed.


input: two plus two
output: four

input: twenty-one point five minus one point five
output: twenty

input: one minus two
output: negative one

input: five times five
output: twenty-five

input: twenty-five divided by five
output: five

input: two plus two minus five times five divided by negative zero point five
output: fifty-four

input: one million times one thousand
output: one billion

input: one million times one thousand plus one
output: one billion, one

input: two million plus one million:
output: three million

input: one million plus one million plus one million
output: three million




Oh, and if you actually going to test it, it may take a few seconds, as it loads the complete Wolfram Alpha page up to two times.  

Well, there might be a lot to enhance though.