What techniques do you use to design and scale a web app horizontally (in EC2)?

Specifically I'm looking for techniques for scaling a web application which has no central database server, in the cloud, but general advice is great.

I have come across GlusterFS, which looks great, but I'm not yet clear how it fits into the architecture of a web application. This also is interesting to me.

Thanks for the advice and links.

From serverfault jberryman

• Of the top of my head I would look at Amazons SimpleDB beeing that you are in the aws ecosystem already. But its hard to say without more information about your application than its just "a web application".

  Tom O'Connor : Amazon's SimpleDB is frightfully expensive. I used it for about 10 hours one night, and discovered that they'd billed me for $50 of usage!?

  jberryman : Thanks, I was hoping for more general discussion about this topic so I didn't include much info about our app. But since no one seems interested... we are a google-docs-style application and (at the moment) are storing user data in individual directories, each with an sqlite DB. We do this because our system will be both on and offline, and users can down/upload their data in this form.
  From intellectdk