Google Drive is currently experiencing some down time but at the moment Google has not commented on what the problem might be with its cloud storage service. The downtime was first reported over an hour ago and at the moment Google's own App Status Dashboard site shows that there is indeed a "service disruption". The site has a message on its Service Details page which says, "Affected users are experiencing slowness, latency issues, and receiving 500 errors when accessing drive.google.com"
At the moment, Google Drive is the only online service from the company that is being affected by this problem. Gmail, Google Docs and Hangouts are not affected. When Google Drive is back up and running we will let you know.
Google has chimed in, confirming the issue, and promising a fix soon:
Google Drive service has already been restored for some users, and we expect a resolution for all users in the near future. Please note this time frame is an estimate and may change.
Source: Google; Thanks to Horatio and everyone else who gave us tips!