Until now, you could switch to Google.com by clicking "use Google.com" at the bottom of the homepage. Google made a change and it now shows the "use Google.com" link only when you're redirected from Google.com to your country's Google site.
This way, most people will no longer see the "use Google.com" link. One workaround is to visit Google.com to see the link. Another option is to go to www.google.com/ncr, the same URL that used to the displayed by Google. "NCR" means "no country redirection", so the URL can be used to bypass Google's redirection.
Why would Google hide the link? Danny Sullivan suggests that the latest change could have something to do with the "Right To Be Forgotten" feature. "Google’s come under increasing pressure from both Canada and the EU to censor its results on Google.com in addition to censorship demands on country-specific versions."
I use Google.com because it has the latest features and it's more up-to-date.