Select multiple lines of code and press Ctrl+K,Ctrl+C, and you hard work is going to be ignored. To reverse just press Ctrl+K,Ctrl+U.
These keystrokes will comment/uncomment your code only with line comment like // in C# or — in SQL. I’m not aware how to comment code with block comment (/**/) without using a plug-in for VS.
One more thing. If you need to comment/uncomment a single line, you don’t need to select the whole line. Just move the cursor to any position on the line you want to comment, and let you fingers press the magic combination.
If you like me (there’s nothing wrong to be different from me) and use ReSharper, than you would like to use Ctrl+Alt+/ with line comment and Ctrl+Shift+/ with block comment.