Are there other "fun" packages like the "coffee stains" package?

Not a package, but could be packaged pretty easily: beamerduck!

The following presentation will show a progress by having a duck walk from left side of the screen to the right side, saying annoying things every once a while. You can of course replace the duck picture by any other animal (I am thinking about a man carrying a sack on his back. On specific slides, the ones I plan to spend more time on, I want to have him drop the sack to the ground and rest).

\documentclass{beamer}
\usepackage{tikz}
\usepackage{lipsum}

% Duck stuff

\usetikzlibrary{shapes.callouts, calc}

% Parameters: file, number of pages, width
\newcommand\DuckSetup[3]{%
\foreach \n in {1,...,#2}{
\pgfdeclareimage[width=#3,page=\n]{duck\n}{#1}}
\def\ducknumberofpages{#2}}

\DuckSetup{duck}{2}{1cm} % "duck" is a pdf file with 2 pages that will 
                         % alternate as they move from slide to slide.

\newcommand\duck{%
\tikz[remember picture]{\node (duck) {%
\pgfmathparse{int(mod(\thepage-1,\ducknumberofpages)+1)}%
\pgfuseimage{duck\pgfmathresult}};}
}

\setbeamertemplate{footline}
{%
\pgfmathparse{(\thepage-1)*\paperwidth/\insertdocumentendpage}%
\hspace{\pgfmathresult pt}%
\duck
}

\setbeamertemplate{navigation symbols}{}

\newcommand<>{\ducksez}[1]{%
\uncover#2{\tikz[remember picture,overlay]{\node[ellipse callout, draw, fill=white, overlay,
callout absolute pointer={($ (duck.north east) + (1,0) $)}] at ($ (duck.north east) + (3,1)
$) {#1};}}}

\newcommand<>{\ducksezrev}[1]{%
\uncover#2{\tikz[remember picture,overlay]{\node[ellipse callout, draw, fill=white, overlay,
callout absolute pointer={(duck.north west)}] at ($ (duck.north west) + (-3,1) $) {#1};}}}
% End of duck stuff

\author{Egon Ipse}
\title{The Importance of Being a Duck}
\begin{document}
\begin{frame}
   \maketitle
   \ducksez<2>{Hello!}
\end{frame}
\begin{frame}
   \frametitle{Purpose}
   \begin{enumerate}[<+->]
      \item Purpose of Ducks
      \item Purpose of Duckweed
   \end{enumerate}
   \ducksez<3>{Quack!}
\end{frame}

\foreach \i in {1,...,10}{
\begin{frame}
   \frametitle{Frame \i}
   \lipsum[\i]
   \ifnum \i=5 \ducksez{Five!} \fi
   \ifnum \i=9 \ducksezrev{Nine!} \fi
\end{frame}
}

\begin{frame}
   \frametitle{The last frame}
   \ducksezrev{See Ya!}
\end{frame}

\end{document}

Hope this counts. Cow font anyone :-)? It's part of TL Contrib.

http://www.tug.org/TUGboat/tb27-1/tb86hoekwater-cows.pdf

I like the chickenize package, which can do a lot of useless things, among others it can print in rainbowcolors. See yourself, works only with Lua(La)TeX.