Since a while I have been using Dropbox with great pleasure. 2Gb space for free, easy to use and some nice features like public folders and sharing files. Also the possibility to install Dropbox on your iPhone, Android or PC/Mac is neat.
Despite those advantages, I was curious about alternatives and I had been using SkyDrive from Microsoft a while.
SkyDrive offers you loads of free space (25Gb) but has no application running on your device or PC. Although drag and drop is available on Windows(7). At the other hand you can make use of some native apps like Sorami on your Android.
Other free alternatives are published on: http://mybroadband.co.za/news/internet/16719-Dropbox-alternatives-Cloud-storage-burst.html
Futhermore (I have no idea if Dropbox has disproved this matter) there are some rumors about the vulnerability of Dropbox: http://dereknewton.com/2011/04/dropbox-authentication-static-host-ids/