Dropbox is a really useful cloud app, but you can normally only run one account at a time. With Dropbox Encore you can run a second account, giving you more flexibility.
Running Dropbox Encore requires Dropbox to already be installed. A second Dropbox icon will appear in the OSX toolbar, and will automatically ask you to sign-in or create a new account (you cannot link two accounts to the same email address).
Dropbox Encore works very smoothly, but there are some acknowledged problems with it. The main issue is that the check marks next to synchronized files do not appear in the second Dropbox. You cannot follow the default setup for the second account, as it's necessary to have a different folder attached, not the normal dropbox folder. However, this isn't difficult to do.
The author warns that running simultaneous Dropbox accounts can be 'confusing and lead to mistakes'. Dropbox Encore is only really necessary if the normal sharing options aren't quite enough for you.
Dropbox Encore is a neat, if imperfect, app that lets you easily run two Dropbox accounts at the simultaneously.