This may sound stupid, but I ran into this issue that upon pressing F12 or choosing Tools > Developer Tools, the IE Developer Tools window does not show up. It is, however, showing up in the task bar.
The issue is that when you have Developer Tools detached from the browser window, and you don’t close it before exiting the browser, the next time you open IE8 and try to run Developer Tools, the Developer Tools window will be an ‘invisible state’ (simply put…).
The solution is to right-click on the tab/ icon in your Windows task bar and choose Maximize. If there is no such option in Windows 7, hover on the tab/ icon in the task bar for a moment until an overlay pops up that shows all the IE8 instances and the Developer Tools representation view, right-click on the Developer Tools’ representation view and choose Maximize. Alternatively, you may do Alt+Spacebar, then hit ‘m’ and start using your arrow keys to move the window around. You will then be glad to see your long-lost friend!
It sounds stupid, but this is still a kind of IE bug.