Google Calendar has three way of notifying you upcoming calendar events and appointments. One is the regular email alerts and the other is pop-up reminders in your browser. The third option is SMS alerts which is ideal when you are on the streets. The pop-up reminder is kind of useless as it requires you to keep the Calendar page open.
Google Calendar has a Labs feature called "Gentle reminders" that replaces pop-up reminders with improved notifications. On enabling this option, the title of the Google Calendar window or tab will happily blink in the background when you get a reminder.
You can also add sound notifications, in case you were not looking. You can enable this feature by going to Settings > Gentle reminders (labs) and selecting Play a sound notification together with reminders (requires Flash).
If you're using Google Chrome, there's another neat trick. Gentle Reminders can access Chrome's built in notifications system for web apps and display persistent notifications even if you're not using Chrome at the time, as long as it's running in the background.
This way, any reminder you've set up will show up in the corner of your desktop ensuring that you never miss it again. The notifications will not close until you do it manually. Most operating systems have notifications that fade out after a period of time. The advantage of persistent notification is that you're guaranteed not to miss it.
Those who are looking for more than notification can try out GMinder, a desktop program that waits in your system tray and alerts you when you have an upcoming Google Calendar event. GMinder supports multiple calendars and allows you to configure how you want to be alerted. Since it downloads your events, it works offline and enables you to preview your agenda of events even when not connected to the Internet. This is an added advantage.
When you get an alert you have the option to either open the event in your browser to view, dismiss it or snooze it for a fixed duration.
GMinder also enables you to add new events to your Google Calendars and comes with a bunch of customizability options.
[Thanks Google OS]