edit: forgot to add the url: http://www.microsoft.com/bizspark/
If you can stand the HN karma deduction of using the MSFT platform, it's a pretty fantastic deal. Oh, and the program administrators are less picky with how "real" you are than, say, becoming a registered iOS developer (having gone through both processes myself).
If I was building a web application then I would avoid MSFT but using MSFT hardware is much easier than developing my own or hacking stuff to work with MSFT hardware.
Not necessarily a good or bad plan, if you're set on using Microsoft technology anyway, you can save licensing costs for the first few years, and minimize some costs should your project not succeed. If you're just testing the waters as to which platform to use, BizSpark will tend to lead you towards incurring additional operational costs in form of Microsoft licensing fees.