Android 4.0+: After using Calendar Status for a few days, I wonder why it didn't exist before. The app puts your Android device's Calendar into your notification drawer. It's the easiest way to check your calendar on Android.
The setup is very easy. Install the app and it will sync with your device's default calendar (not your Google Calendar or any other calendar app you use). That's perhaps its only shortcoming as you can't use some of the alternative calendar apps for Android. But if you've set up your Calendar to sync with your Google Calendar, which is normally done on most Android devices, then it shouldn't be a problem.
Pull down your notifications pane and Calendar Status will show you upcoming events in your calendar. There are plenty of options to customise the display. You can set background colours, event colours, styles, number of events, merge events on the same date, and much more. The paid Pro version even lets you pick which calendars to show, gives an expanded layout for today's events and offers a few other options. Try it before you buy, but it's well worth it.