Here is a list of few important free resources on ASP.NET:
1) ASP.NET Resource Kit from Microsoft: These are set of web controls from www.componentone.com, that Microsoft has made available free of cost. It includes Controls for Web Menu, Web Chart, Web Report, Web Data Objects, Web Bars & Paypal ecommerce. Download from here
2) ASP to ASP.NET Migration: There are millions of webpages developed on ASP. If you want to convert your ASP Pages to ASP.NET, Microsoft recommends you use their ASP to ASP.NET Migration Assistant. Read more on this at MSDN ASP to ASP.NET Migration Section or at ASP.NET Migration Assistant page.
3) Internet Explorer WebControls References: These are some cool controls that work with Internet Explorer exploiting IE’s enhanced DOM model. It includes Multipage, TabStrip, ToolBar & Treeview ASP.NET Webcontrols. You can read the reference here and download from here.