Nudgemail is the clever service that redelivers emails you send it as a reminder. Now it's doubly helpful to send NudgeMail your follow-up-needed items, as it can add those emails to your Google Calendar.
As NudgeMail explains in its blog post, once you've authorised the service through Google Calendar, it can automatically add those emails you set for redelivery to your calendar. So when you forward an email about cleaning your desk to [email protected], you get both an email on Friday reminding you of this, and a Gcal appointment for that Friday. The drawback is that you can't modify or remove items from NudgeMail on Google Calendar itself — you have to email [email protected] and remove events through their email-based system. But use NudgeMail only for things you really want on your calendar, and you're all set. NudgeMail is free to use, and doesn't require a sign-up, just verification through your email address.