Note: This version is for Visual Studio 2010. For other versions of Visual Studio (2012, 2013, 2015 or VS "15"), please go here.
Note: This extension has also been bundled in the Visual Studio 2010 Pro Power Tools extension where it is called "Column Guides".
"Editor Guidelines" is an editor extension for Visual Studio 2010 which adds vertical guidelines behind the text at user-specified column positions. It emulates an undocumented feature from previous versions of Visual Studio. That feature was removed in Visual Studio 2010 but, by downloading and installing this extension, you can bring it back. See also this blog post.
This extension has no built-in user interface, but I have another extension, "Editor Guidelines UI" which adds new commands to the context (right-click) menu in the editor to give you an easy way to manage the guidelines.
Without the optional UI extension, you will need to edit the registry to make the guidelines appear in any text document. Note: You still need to download and install this extension before the registry edits will do anything.
For Visual Studio 2010 the key is:
Add a string (REG_SZ) value called "Guides" and change its data to (e.g.) "RGB(255,0,0) 4, 29"
The RGB(...) part is for the color of the guideline (Bright red in this example). The values following the color are the column values for the guidelines. You may have up to 12 guidelines. In this example, two guidelines are placed. One after column 4 and one after column 29.
The following screenshot from regedit.exe shows setting a single, dark red guideline at column 80: