Dear Author’s Jane has a great article on how to set up your own cloud of ebooks. You need Dropbox (free) to set up the cloud, Calibre (free) to set up a library in the cloud and the Calibre Open Publication Distribution System (free) to manage the cloud.
Her instructions are for Windows, but if you check the comments you will see a link to instructions for the Mac and a lot of other helpful tips on how to get everything working. I don’t have any need for this, but I do use Dropbox on a regular basis to share files with my daughter. It always works like a charm.
UPDATE: This method has since been superceded by calibre2opds.