We assume that "reminder" and other alerting/notification capabilities are a given since they are so entrenched in existing collaboration and communication tools (e-mail and calendaring) - but such functionality is not always there when it comes to more recent social software tools (such as tag/bookmark tools):
Launched over the weekend as part of a one-day-startup party, Tagmindr links up with your del.icio.us account and lets you easily and automatically send yourself bookmarks in the future. Simply signup and then tag your del.icio.us bookmarks with the tagmindr tag and a date in the format of "remind:YYYY-MM-DD". Tagmindr will then put your bookmark in an RSS feed on the date you indicated. Eventually they may support SMS, Email and IM. But it probably wouldn't be wise to hold your breath.