I quite like the functionality of rememberthemilk, an online to-do manager. I find it very straightforward to add, modify, and prioritize tasks. I really like the flexibility in entering dates. For example, you can just enter ‘tomorrow’ or ‘Friday’ and it will input the proper date for you. There is additional functionality to have reminders sent via SMS and email. Calendars/Lists can be shared with other contacts and you can even subscribe to them via iCal on the Mac or via an Atom feed.
I am still exploring what rememberthemilk can do. Everything that I have seen thus far is most impressive and truly useful.