Dropbox. It's the Apple of Web storage companies. It's got a clan of fanboys who swear by it. And for good reason: It's simple to use and has some strong features. It's also one of the least flexible products among its competitors.
Holdouts (like me) who want the capability to sync files outside of Dropbox's walled garden -- the Dropbox folder on your desktop -- have to look to other products. Likewise, people who want to store or sync more than 100GB of files.