More and more often, it becomes obvious to me how important an external API is. What better way is there to share your product, then to let people build on top of your service layer and customize their own interface? If someone doesn't like the look of your program, or how it feels, they may leave; but a missed opportunity can quickly turn into another user when they find someone else's implementation of your application. Or, other applications may pull your data into their product, giving you another revenue stream (or just more users), more popularity, your name thrown around more; it's free advertising and a great testimony to your product that someone liked it so much that they included it in their product.
I recently looked into Flickr's API, for integration into a project I was working on, which spawned this thought. It is absolutely fantastic that I can call a URL with some credentials, and bam, I have the file I need. The user doesn't have to upload (or worse, download from Flickr and reupload) to another photo-sharing application; they can just point out a url, and I can display it. It's really awesome. Twitter has feeds, that I can pull into my blog; even my blog has an API that will allow other sites to post comments to me without ever loading my site. That's pretty cool stuff.
For an information-hosting web app, this is one of the best things you can do to increase views and popularity. If you have a good service layer, expose some of it (with the right security, of course), let other people use your app. More users is never a bad thing.