I am good at coding in .NET and I build some my own websites when I am free. Also I want to start a small organization for software development.
In recent days I have found so many open source technologies like NOSQL, HBase, MapReduce and other stuff that support to build scalable applications.
Is it good to learn completely new technologies having had 4 years of experience in .NET? Will I get any real benefit if I learn new technologies while ignoring .NET?
Please suggest me.
It's always good to learn new things, but that doesn't mean you have to ignore .net. For sure you'll learn interesting things and different approaches, and maybe it will help you solve problems (even .net problems) in different ways.
There are a couple of .Net Mongodb providers and mongodb runs on windows. Read http://www.mongodb.org/display/DOCS/C+Sharp+Language+Center.
You can also learn IronPython, F# or IronRuby if you want to learn a new language. You don't have to relearn everything, you can, but you don't have to.
One of the benefits of using open source technologies is cost. If your primary goal is to start a software development business - you may want to consider the cost of deploying your solutions to your clients.
If you are developing in .NET, you need to make sure that your client has the server software and licenses to support your product such as Windows Server, SQL Server - all of which can add additional cost to your software solutions.
Open-source technologies deployed using the LAMP model often will not require additional costs to your clients.