Google Calendar has a Labs feature called "Gentle reminders" that replaces pop-up reminders with better notifications. "When you get a reminder, the title of the Google Calendar window or tab will blink in the background." If you use Google Chrome, you can also enable desktop notifications in the settings.
Google has recently added sound notifications. You can enable this feature by going to Settings > Gentle reminders (labs) and selecting Play a sound notification together with reminders (requires Flash).
[Source, via Alex]