While there are a number of good tools out there for working with scalability, more important than any particular tool is building your application in such a manner that it's easily parallelizable. In a Web app, a core principle to keep in mind is that the more stateful the application server-side, the more difficult it is to scale, and so designing your application tiers in such a way as to decouple requests is key. Limit the amount of session state the server has to keep track of, and you'll be able to load-balance request handling smoothly.