Backpack is a genuinely brilliant site. It is essentially an online personal information manager, giving you the ability to maintain an online diary or keep todo lists, posts photos and all sorts.
Steve Rubel explains it far better than me.
Backpack lets you create and share lists, photos, documents and more in a wiki-like editable Web site you can share – like this one. I am trying it out to share blog links with my client and I love it so far, though I wish it enabled RSS feeds for each page. I see lots of applications here for the PR community including instant press rooms that can be set up in the event of a crisis. Kudos to 37 Signals. It’s also generating a ton of buzz. I bet they’re going to give Jot and Socialtext a run for their money at least in the small biz market.