Since Google Analytics was revised the currently accepted answer is no longer available. The current version of Google Analytics will let you view visitors per hour:
- Go to the Reporting Tab
- Select “Audience”
- Select “Overview”
- Select “Hourly”
If you’re trying to determine total traffic by hour of the day for a given date range then you can use this custom report:
https://www.google.com/analytics/web/permalink?type=custom_report&uid=pToP7693ReSxfSIpbqHmWg