Try RememberTheMilk. I've been using it for years and highly recommend it. The killer feature IMHO are the smartlists that you define by logical arguments, eg.

Try RememberTheMilk. I've been using it for years and highly recommend it. The killer feature IMHO are the smartlists that you define by logical arguments, eg. "tag:home AND priority:1 AND dueBefore:tomorrow" creates a list of tasks that autoupdates as soon as something changes.