You can design a birthday card, anniversary card, etc. attach it to an email a few days prior and have the email sent to that person on that special day. You may forget to send it but Boomerang won't.
While you can use Google Calendar for reminders, you also can use Boomerang as an email reminder service. You can set an email to be sent to you reminding you of a meeting, to pay a certain bill, etc. Further if you receive an email confirming an event, you have the ability to send it to Boomerang. Boomerang will remove it from your inbox and put it there at the time you designate to remind you of the event. If the confirmation mentions a date and time, Boomerang will suggest a date and time to send the reminder.
You can also set up a reminder for you to follow up on a message you send. You can set it to remind you to follow up if the person you sent it to does not respond or regardless if the person responds or not. The reminder will go to your inbox at the top and is marked unread. You also can use this feature for a cleaner inbox. Boomerang will store a message on their server and return the email to your inbox at the time you note that you need it.
Go to http://www.boomeranggmail.com/ to download the program.