Techcrunch had an article with 13 of them... http://www.techcrunch.com/2006/01/31/the-online-storage-gang/
It's a pretty crowded space. And XDrive gets you 5 GB for free, 50 GB for $9.95 a month. I can't expect Dropbox to charge those prices, given S3 as a backend. The margin just isn't really there, especially given the number of uses that will want free storage. And I think competitors can duplicate Dropbox's nice front end. In fact, here's an open source front end to SVN which is similar to Dropbox's:
http://tortoisesvn.tigris.org/
Sorry for all the negativity, I guess I'm trying to play devil's advocate here. It's a wonderful product you got going there, but I think you will have to work really hard.