How to typeset boldface ell?

You can use \boldsymbol from the amsbsy package; if you are going to use the symbol very often, then you can define a new command, as in the following example:

\documentclass{article}
\usepackage{amsmath,amsbsy}

\newcommand*\Bell{\ensuremath{\boldsymbol\ell}}

\begin{document}

\Bell

\end{document}

\documentclass{article}
\begin{document}

$\ell$\boldmath$\ell$\unboldmath

\end{document}

I could only get it to work using:

\usepackage{bm}

\newcommand*\boldell{\pmb{\ell}}