I've just found dozens of them at http://www.download.com/
Just search for 'alarm clock'. When the first page of suggestions comes up, scroll to the bottom of the page and change 'licences' to 'free', then click 'update'.
For something more advanced, repeat the process but searching for 'scheduler' insted of 'alarm clock'.
You could schedule things "to do" into a Yahoo! calendar and set it to send yourself an email 5 or 15 minutes [or whatever time you choose] before you need to do something.