printing passwords: How can I differentiate l/1, o/O/0, etc and visualize spaces?

As @uSlackr said, eliminating the troublesome characters helps a lot, if you are generating these yourself.

That said, you'll want to look at fonts optimized for programmers, because they run into these kinds of problems all the time. Everyone's got an opinion, so Google for "programming fonts" and you'll finds lots of pages with samples to view, like this one. I'm very partial to Consolas myself:

Now, one thing you can do to help out with the visible whitespace is use Notepad2 as a text editor. (I use the Notepad2-Mod variant myself.) It has an option for visible whitespace, so tabs are denoted with an arrow and spaces with a small dot in the middle of the character space. This isn't just for display though, these will also print out. Below is a snapshot of a PDF created by printing from Notepad 2 with visible whitespace and indentation guides on:

All other Scintilla-based editors probably behave in the same way.

You could do it like banks print their PIN numbers - both normally and phonetically.

So the password 'h30yg28fi1' would be also written:

AICH THREE ZERO WHY GEE TWO EIGHT EFF EYE ONE

or something similar.

That would differenciate "oh", "OH" and "ZERO", "eye", "EYE", "ell", "ELL" and "ONE" quite well.

Sorry for the late feedback. I used this solution in the end (bitstream vera font in latex with verbatim*):

\documentclass{article}

\usepackage{bera}

\begin{document}

% text using bera font (Bitstream Vera), which has dotted zeroes
% the * after verbatim adds visible spaces
\begin{verbatim*}'1 l oO0}4 _i6D-WtF#q\end{verbatim*}

% visible space
hello\textvisiblespace world

% slashed zero
\o

\end{document}

I left a few of the other things I tried as well.

To create a pdf from it:

pdflatex password_printing_4.tex

If you get any errors, try simply installing texmaker. It depends on most necessary latex packages:

sudo apt-get install texmaker

I also tried the inconsolata font in latex, but it doesn't seem to be the same:

\documentclass{article}

\usepackage{inconsolata}
%\usepackage{bera}

% requires texlive-xetex package:
\usepackage[xetex]{graphicx}
%\usepackage{fontspec,xunicode}
%\defaultfontfeatures{Mapping=tex-text,Scale=MatchLowercase}
%\setmainfont[Scale=.95]{Inconsolata}

\begin{document}

% text using bera font (Bitstream Vera), which has dotted zeroes
% the * after verbatim adds visible spaces
\begin{verbatim*}`!=}k o6~-cRR1loO0\end{verbatim*}

%\renewcommand{\bera}{\inconsolata}
\texttt{This is Inconsolata. zero: 0}

%\renewcommand{\inconsolata}{\bera}
\texttt{This is Bera. zero: 0}

% visible space
hello\textvisiblespace world

% slashed zero
\o

\end{document}