Next time when you can't access Gmail, Google Calendar is down or Google Docs loads very slowly, visit the Google Apps Status Dashboard. Despite its the name, the page shows the latest status data not just for the Google services that are included in Google Apps, but for the corresponding consumer services as well.
"[The] page offers performance information for Google Apps services. Unless otherwise noted, this status information applies to consumer services as well as services for organizations using Google Apps."
Today's status for Gmail informs that "a small subset of users" couldn't access Gmail and that the service has been restored for most of the affected users in about 3 hours. "The rest of the users should be coming back online within the next 24-36 hours."
Looking back at the historical data, it's clear that the Google application which has the most issues is Gmail. Another Google service that shows a detailed status dashboard is Google App Engine and it would be nice to show similar information for other Google services.