Dropbox and SugarSync are services that you can use to store your files on the cloud so that you can retrieve them from any location and device with an Internet connection. They do what iCloud should do. Dropbox is the most popular, so most apps have a dropbox integration.
I don't know about others, but for me the one big problem with iOS is how it won't give you access to your files to do as you want and exchange between apps. To open a file from an app in another app, you have to use the "open in" feature, and that has to be supported by both apps (plus it duplicates the file). The frustrations are endless, but GoodReader, with dropbox or another cloud service, can help some