Over the past week or so, I've been trying out Backpack. It's rather difficult to describe exactly what it does, and the best way to learn is by trying it yourself (you can sign up for a limited free account). However, by just saying "Go and try it", I feel that I'm failing in my duty to the geek community, and being pretty lazy into the bargain. So here's a slightly more informative discussion.
Backpack allows you to create and link together a number of web pages, which can contain text, check lists, notes (blog-like entries with a title and a date stamp), links to other pages and tags (like del.icio.us tags). In addition, if you have any of the account plans other than the free one, you can also upload images and files. Backpack uses Ajax extensively to handle creation, deletion and editing of elements, so it feels very snappy and smooth to use as you rarely have to refresh the page to see your changes.
So far, it probably sounds like a nicely implemented wiki, but there are a few other nice features that make it much more useful than that. First, you can send items to a particular page via email. Each page has it's own unique and randomly-chosen email address, and sending an email to that address with 'todo: Buy flowers' puts a check list item 'Buy flowers' on that page. Using a similar notation, you can add images, notes and files to the page. This is pretty brilliant, and I've used it several times already from my mobile phone when out and about. I used to use Ta-da Lists to house my wishlists of books, CDs and DVDs that I would like to get for myself or as gifts for others, but Backpack provides everything Ta-da did and more so I've switched. The emailable items become very useful when you're in a shop and spot something you'd like to get at some point. You can make a quick entry there and then, and then relax knowing that it's safely on your list.
Reminders are also very handy in certain situations. You can set up reminders for any date and time in the future, and the text gets emailed to you or sent as an SMS to your mobile phone. I'm finding this useful for things like 'Return library books' which don't really belong on my calendar, but I need a reminder for, and also to remind me to take something to work. I review my to do items (kept in Tracks) regularly, but I don't get time to do it before work. A quick SMS reminder timed to arrive while I'm having breakfast is an excellent prompt to remember to take something with me, or do something on the way to work.
Finally, there's now a mobile interface to all the pages, which loads very quickly and cleanly on my phone. That means that you can read and add things to pages, and even add reminders while on the move.