I'm beginning a project with a client to build a web application and I'm a little stuck on which solution to go with.

I've used Joomla for many clients in the past, but this client has specific requests that I KNOW I'm going to have to build myself.

The problem I'm facing is that I work full time under the .NET spectrum and while I am a novice developer in PHP, and I've been studying Joomla's plug in architecture for about a month now, I am a lot more comfortable building something in ASP.NET than I am in PHP.

My question is, what OS projects are out there that have a similar community following as Joomla/Mambo/Drupal, along with a plug in architecture that is somewhat akin to Joomla as well?

I don't really have the time to build out a full blown CMS system in ASP.NET, but if something already exists that can give me X% (25%, 50%, something) of what Joomla has that will at least get me on the right path. Joomla just has too many extensions and too much of a community backing for me to pass it up if there's not something comparable in the ASP.NET realm.