You can also expand your Dropbox storage space by sharing your special link with friends. This is my special link: http://db.tt/WFnvRY0y - if someone signs up using this, I get more storage space. Simple. There is of course a maximum, but that's ok. Go ahead and sign up using my link if you haven't already, then share your link with someone else to get more. We could all benefit from a little more, and from helping each other out.
Lastly, you can also share a folder from Dropbox with other collaborators to provide access to your files. That folder will also be synchronized to their computer. Too easy, right?! Just be careful of virii, etc.