Side note: I had this same idea years ago and launched a prototype but never got enough traction. Then Contactually came along and took the market by storm. Just goes to show you that it's all about the execution and not only the idea, so kudos to them.
One, at http://www.hassleme.co.uk has a free, simple and painless email reminder system - "we'll nag you via email at semi-unpredictable intervals" (of N days, plus or minus a few).
I think you'd find this site useful: http://contactually.com
Side note: I had this same idea years ago and launched a prototype but never got enough traction. Then Contactually came along and took the market by storm. Just goes to show you that it's all about the execution and not only the idea, so kudos to them.