Even if your site is still months away from being ready for launch, you should put up a “coming soon” page as soon as possible. A “coming soon” page can help you collect user information, like email addresses and feedback/suggestions. It can therefore help you build buzz and a base of early adopters prior to your site’s launch.
For ideas and inspiration, check out these “coming soon” pages. (They used to be called “under construction” pages, an archaic term from the early days of web design.)