If you develop high scale, scale out/distributed apps, check out

Microsoft Project Code Named “Velocity” Community Technology Preview 3 (CTP3) Released to Web on 7 April 09.

"Velocity" is a distributed in-memory application cache platform for developing scalable, high-performance applications. "Velocity" can be used to cache any common language runtime (CLR) object and provides access through simple APIs. The key aspects of "Velocity" are distributed cache performance, scalability, and availability.

This release contains several security and performance enhancements, as well as the new cache notifications feature which allows clients to be notified when changes are made at either the key, region, or cache level. In addition, it also includes numerous refactorings to its API, making it more conformant to the .NET Framework guidelines as well as easier to consume. “Velocity” is currently slated for release in the middle of this year and will represent an integral piece of our overall web/data story.
When complete it will be part of the .NET platform, ie: Free