Recently Microsoft Velocity product was announced at TechEd. Other than the vague name, this product seems a good fit with .net solutions where heavy caching is required in a clustered environment over many physical servers (or just one). This product seems highly configurable and easy to use. This will simplify the session management in ASP.NET solution as well as allow developers to load lookup data from database or any back end stores instead of using ASP.NET caching or singletons. With high scalability, and reliability over distributed architecture, this product definitely a good one to have in your toolbox. Remember Velocity is at its very early stages and it is going to evolve with community feedback.

Managing session state over web farms usually is configured with some backend database servers to provide centralized persistance, but this is slow compared to accessing the same data from memory and you should be able to get significant difference in performance.

It would be nice if this caching architecture provided a asynchronous backend data persistance capability. This will allow us to load large tables that are often used for lookups but rarely updated in order to avoid updating them seperately and invalidating the cache.

Here is MSDN page for Velocity and a pod cast from hanselman.com along with many other resources.