I wanted to have some fancy boxes for displaying particular definitions - in my case, formal definitions - that would be readable, span pages, and easy to use. I wanted a left-hand margin border and background shading for the entire box.

After much searching and modification, I found that the framed package worked well, after some modifications. I can then use the following Latex code:

Some text goes here.

Which will produce an output similar to that at the right.

The definition for the formal environment is as follows:

% for adjustwidth environment

% for formal definitions

% environment derived from framed.sty: see leftbar environment definition

    {\color{darkblue}\vrule width 2pt}%
    {\color{formalshade}\vrule width 4pt}%
  \noindent\hspace{-4.55pt}% disable indenting first paragraph