The shorter version of what the much-loved Backpack, the new information organiser from 37Signals, can do, which is already being done by Gmail.
Add content to a (password protected) page via email: Can be done in Gmail
Send content from a (password protected) page via email: Can be done in Gmail
Funky AJAX interface: Strange, it is there in Gmail too.
Tagging for grouping information: Also present in Gmail as labels.
Essentially, every Backpack page is a media-rich page or for the lack of a better term - a rich wiki - which does not do, other than the done-to-death reminder feature, much more than what Gmail already does for me. I have a Gmail account which has the labels todo, work, ideas and raw that filter my emails to that account when sent with the label: text as the subject line, which handles all my notes and ideas. Forward the same mail to myself and hey I have the option to have versions too. And as far as the collaboration features of Backpack goes, they are just not strong enough or comprehensive enough for me to even consider using it.