Besides of the above URL, http://downforeveryoneorjustme.com/ works not only for google but any other service too. However, I got suggested that tool via twitter, but I have no clue how it works or how reliably it is.
"Looks down from here" suggests to me that it's simply doing an HTTP GET and interpreting the returned status code (or timing out). http://downforyouaswellorjustme.com/ would probably be more accurate.