Drag the files / folders from Windows Explorer into the Solution Explorer. It will add them all. Note this doesn't work if Visual Studio is in Administrator Mode, because Windows Explorer is a User Mode process.
This is originally from Sara's blog.
It also works with almost any version of Visual Studio, you just need to change the "8.0" in the registry key to the appropriate version number for your version of Visual Studio.
The guide line shows up in the Output window too. (Visual Studio 2010 corrects this, and the line only shows up in the code editor window.)
You can also have the guide in multiple columns by listing more than one number after the color specifier:
RGB(230,230,230), 4, 80
Puts a white line at column 4 and column 80. This should be the value of a string value Guides
in "Text Editor" key (see bellow).
Be sure to pick a line color that will be visisble on your background. This color won't show up on the default background color in VS. This is the value for a light grey: RGB(221, 221, 221).
Here are the registry keys that I know of:
Visual Studio 2010: HKCU\Software\Microsoft\VisualStudio\10.0\Text Editor
Visual Studio 2008: HKCU\Software\Microsoft\VisualStudio\9.0\Text Editor
Visual Studio 2005: HKCU\Software\Microsoft\VisualStudio\8.0\Text Editor
Visual Studio 2003: HKCU\Software\Microsoft\VisualStudio\7.1\Text Editor
For those running Visual Studio 2010, you may want to install the following extensions rather than changing the registry yourself:
These are also part of the Productivity Power Tools, which includes many other very useful extensions.
Best Solution
There is a select box for this at the bottom left of the editor window - choose 100% ;)
I was unable to find a keyboard shortcut for it, though zooming in and out can be done using Ctrl + > and Ctrl + <.
Please note the horizontal scroll bar must be turned on to see the zoom level.
Tools / Options / Text Editor / All Languages / Scroll Bars
Another option (Visual Studio 2013/2015) is to use Ctrl with the mouse wheel (up to zoom in, down to zoom out).