Well, maybe not obsolete, but definitely less integral for the mobile professional.
Dropbox syncs your files between computers on which you have their lightweight software installed. It creates a folder where you can store all the to-be-synced files, or you can specify at installation an alternative folder. Anything you “drop” into the “box” gets synced right away to the web interface and any other computers you have running the software and logged in. RIP, USB drive!
Dropbox is a great tool if you use multiple platforms regularly. It works across Windows, Mac and Linux. At home I have an Ubuntu laptop, at work I have a Windows laptop, and I have a persistent Kubuntu USB drive I occasionally use.
I’m still using their free 2GB service, but I frequently toy with the idea of upgrading to the next level: $10/month for 50GB. I would love for there to be some middle ground, say $5/month for 20GB. I could probably justify that expenditure to myself.
I highly recommend giving the service a try, for anyone who is using more than one device to store and work with their data. If you use Dropbox already, let me know why you like it in the comments.