I am noticing that del.icio.us has become much more than a social bookmarking utility for me. I use it far less for searching or research, and more as a repository of my browsing experience. It has become my web memory.
I now wish there were ways I could do the same (i.e. search, notice, relate, tag, recall) with my offline experiences. In offline experiences, I notice that I try to accomplish this with conversations. When I read an interesting article in the newspaper, I turn to an office-mate and comment about it. The dialog then forces me to think a few steps further, add context to the topic, and as a result, I suspect, I can remember the topic better. There is probably some neurological learning research that validates this notion.
With the current buzz about the mobile-phone-as-an-input-device issue, the ability to scan some text (or image, or record an audio file), tag, and store it in a searchable format (in, say, del.icio.us) would be a killer app.