RemindMe is a project mainly written in Python, it's free.
Just add [email protected] to your friendlist in gmail and start using the service. To start just type 'hi' or 'help'.
If you want to be reminded it once at some time, do following.
remind <time> <message>Following will create a recursive reminder.
remind rec <count> <time> <message>e.g.
* remind 10hour pay phone bill ---- It will remind you after 10hours to pay your phone bill
* remind 20:00 book movie ticket ---- reminds you to book ticket at 8PM
* remind rec 5 11:00 project meeting ---- reminds you at 11AM for next 5days for project meeting