This style for Google Calendar puts a horizontal blue line on the calendar grid at your chosen hour and/or half-hour position(s). The default is one line at noon** (which you can move or add more to by editing the code).
[ http://freestyler.ws/style/27847/gcal-horizontal-blue-line ]

(**Why noon? It's the middle of the day, and my Calendar grid shows 13 hours at a time, so noon is the only hour of the day that's always showing, whether it's scrolled all the way up or all the way down--the line is always visible... makes it easier to know at a glance if I'm looking at the A.M. or the P.M. for those not-seldom-enough times when Calendar randomly shifts the calendar grid on me coming back from settings or event creation/modification or plain old navigation.)