I was trying to add AJAX Control Toolkit to my Toolbox, for some reason Visual Studio was crashing. I found a solution for it, see this works out for you – start Visual Studio in safe mode (go to Visual Studio’s command prompt and execute ‘devenv /safemode’) and then try to add items to your toolbox, I was able to do it! There might be a better solution for this, please let me know if there is one.