I thought I should update this as there have been a few developments in the realm of ASP.Net application infrastructure. Namely...
This is, amongst other things, an ASP.Net session state provide which works by distributing session state across multiple servers in a cluster.
But it also supports safe shutdown of cluster nodes as it will automatically migrate session data to other nodes. So you can get rid of stick sessions.
It is an in process session state provider. But within it's own process, so it supports web gardening.
It also has an optional repository database schema you can implement should you want state/process information to persist beyond an individuals session.
AppFabric can write any serialisable object away to it's database without having develop a custom process.
It also has some amazing features for IIS hosted WCF & WF services (a.k.a Workflow Services) but I'll leave those for another day.