No, not that one. Your online Backpack. Backpack, by 37signals, is a personal information manager from the makers of Basecamp (a project management web app) and Ta-da List ("simple sharable to-do-lists"). It launched today, and I'm pretty impressed. Basically you create a project page (and this is 'project' in the broadest sense of the word) to which you can add a to-do-list, notes, files, images, and links to other project pages. You can also set email and mobile reminders, so you won't forget an appointment, a deadline, to return your books to the library, or to get a loaf of bread.
You set all this stuff yourself, so you can manage whatever you want to manage really. Because it is so flexible, it transcends traditional document structure and gathers stuff that normally would be in several documents/folders instead of one page. The example page is pretty inspiring. Personally, I'll use it to keep track of stuff for my thesis. I'm setting up a main page with the most important documents, notes, appointments, and deadlines. I'll also set up a separate pages to maintain my working bibliography, links and research notes. I'll let you know how it works out for me, but I can already say that the wonderful user interface makes it a breeze to set up. Genius. I should probably mention that you can set just about anything in Backpack via the web interface, but also with email. Email enabled cellphones become very tempting once you can use them to set reminders and add to your pages.
Basecamp comes in several flavors one of which is the very nice flavor 'Free'. With the the free version you can't upload files/images and the numbers of pages and reminders you can create is smaller. But hey, free. Recommended.