DotNetNuke is the fastest growing Web Famework with over 500,000 instalations already. The best thing is that it is an Open Source. Its simple to install, seamless and secure. it has a support for free programs that plug in such as:

Maps
Blogs
Forums
Calendars
File Managers
Online Stores

and hundreds more modules available online plus it is SEO optimized as well.
DNN customization ,  DNN module development ,  DNN skin .