I am trying to fix this from last two 2 or 3 days but failed. VS 2010 toolbox in asp application development(design mode or source) is not showing all controls(showing html
This worked for me..
How to completely reset the Toolbox and enable Visual Studio to rebuild it from scratch
Close Visual Studio;
Open the "c:\Users\\AppData\Local\Microsoft\VisualStudio\10.0" (Windows 7) folder and remove all the .TBD files;
Run the "regedit" tool. For this click the "Run" item in the Start menu and type "regedit" without quotation marks;
Find the "HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\10.0\ToolboxControlsInstaller_AssemblyFoldersExCache" and "HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\10.0\ToolboxControlsInstallerCache" keys;
Remove everything from these keys leaving them empty;
Run Visual Studio again and wait until it recreates all items in the Toolbox.