Ewizmo is a free tool you can use to send yourself "reminder" e-mails. It offers the added functionality of tagging these e-mails with labels, which fits in well with Gmail accounts.
Why send yourself reminder e-mails? If you're at work, and you don't have access to your personal e-mail account, you can open up Ewizmo, and send yourself notes to home for stuff like, "wash the dog", "Animal House, 8pm, HBO", or "delete porn". It's useful in places where employers have tight controls on company e-mail usage.
Even if you don't use Gmail, you can still make use of the label feature by creating folders in your e-mail client, and setting up rules that search for label text.
Interestingly, the Ewizmo website itself is sized into a narrow column, making it PDA and cellphone friendly, or allowing you size the window down really small.