Also, I recently found an interesting service Monkey on Your Back that lets you send email “monkeys” that remind other people of their tasks. Unfortunately, other users can also send you reminder monkey, so watch out for monkeys on your back! ;) (A free account only lets you have 5 active monkeys at a time.)
Note: This is just an archive post. The blog has moved to a new home at blog.ialja.com, where you will also find a copy of the entire blog.