The coolest thing about I Want Sandy is the "groupware" function -- if I CC you and Sandy on a message with a reminder, she'll remind both of us. No permissions, no groups, just CC in regular email. The service is free and live and open to all comers.
Sandy's a real Boing Boing-pal effort. It was invented by Rael Dornfest, the former CTO of O'Reilly Media, who (among other things) created the open-source blogging tool Blosxom and chaired the O'Reilly P2P and Emerging Tech conferences. Sandy herself is based on Tim O'Reilly's stellar personal assistant, also named Sandy. And bonus -- the little Sandy logo was designed by our own Mark Frauenfelder! The functionality in IWantSandy is really geared to personal-productivity freaks like me, who were inspired by books like Getting Things Done and the Life Hacks movement.
(Disclosure: I am proud to serve on the advisory board for values of n, the company that produces I Want Sandy)
See also: Meet Sandy -- free email assistant