Open Source Challenger to Dropbox and Box.net: ownCloud

“ownCloud lets you choose where your files are going to be hosted. You can use Amazon S3, you can use Google, or you can drop in your own server. For casual users, ownCloud is probably a bit more maintenance than the average user is going to want to deal with. Folks who are particularly privacy conscious, technical or already running their own servers (or using S3, etc.) will probably take to ownCloud, but it’s mostly businesses that will find this feature particularly compelling.”