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I am using and learning .NET since my coming to telerik. That would be more than a year. If I had to pick a single tool that has been indispensable throughout that time, I would choose NAnt. It is a build automation tool with an XML based syntax, similar to Apache Ant. It has a host of useful tasks, that can make the building of a .NET assembly a piece of cake. NAnt has a sister project -- NAntContrib. It packs some Windows specific tasks, that are not directly NAnt related: IIS virtual directory manipulation, VSS file operations, etc.
*ant tools are not just build...