I contract with a few different companies at a time. I have email addresses with several of them. Each have their own clients who want to share assets - things like comps, artwork, wireframes and so forth. I've played with a variety of ways of sharing these assets. You can attach them to tickets in your ticketing system, but you may not want your client to be in the ticketing system. I've also had clients that were savvy enough to use a repository like GIT or SVN - but that is fairly rare. In comes Dropbox. Dropbox synchronizes and version controls on drag and drop. Almost any client can wrap their heads around that concept.
This works fabulously for one dropbox. However, if you are in a situation where you need (want) multiple boxes, things get a little squirrelly. Dropbox does not support this. Luckily, if you are a mac user, a little script in your terminal window will fix this lickety split, allowing you to have as many drop-boxes as email addresses you work with.