Palatino, Bera, and AMS Euler
I have really been enjoying AMS Euler as a math font lately. If you need completely free fonts, then I think that Palatino + Euler + Bera Serif/Sans/Mono for code, etc. is a pretty workable combination. (This is essentially the default combination used by the ClassicThesis package.) Both Palatino and Euler were designed by Hermann Zapf and work quite well together.
This combination can be used in almost any LaTeX installation, making it a viable combination for the arXiv for example. One must be a little careful to scale the Bera to match the x-height of the Palatino though, or it looks quite strange, but once this is adjusted, the combination looks quite reasonable. (Palatino Sans would probably be the best match, but is not "free".)

\documentclass{scrreprt}
\usepackage[scaled=0.88]{beraserif}
\usepackage[scaled=0.85]{berasans}
\usepackage[scaled=0.84]{beramono}
\usepackage{classicthesis}
\usepackage[T1]{fontenc}
\usepackage{mathpazo}
\linespread{1.05}
\usepackage[T1,small,euler-digits]{eulervm}
\newenvironment{note}[1][Note:]{%
\par\vspace{0.5\baselineskip}%
\sffamily\small\linespread{1.05}\selectfont
\noindent\ignorespaces%
#1
}{%
\vspace{0.5\baselineskip}%
\par\noindent\ignorespacesafterend%
}
\usepackage{listings}
\lstset{basicstyle=\ttfamily,breaklines=true}
\setkomafont{disposition}{}
\setkomafont{section}{}
\titleformat{\section}
{\usekomafont{disposition}\usekomafont{section}}
{\llap{\textsc{\MakeTextLowercase{\thesection}}\hspace{0.7em}}}
{0pt}
{\usekomafont{disposition}\usekomafont{section}\spacedlowsmallcaps}
\newcommand{\I}{\mathrm{i}}
\begin{document}
\chapter{Introduction}
\section{Palatino and Bera with Euler}
This style use Palatino for a body font, with $AMS\ Euler$ font for math: $y =
\sin(x)$:
\begin{equation}
y = \int_0^x\cos(x)\,\mathrm{d}{x} = \frac{e^{\I x} - e^{-\I x}}{2\I}
\end{equation}
Both fonts were designed by Hermann Zapf and work quite well together
as long as you use the small characters --
\lstinline|\usepackage[small,euler-digits]{eulervm}|. Code listings, etc. can
be typeset with \texttt{Bera Mono} to give it a slightly different feel. Again:
you must be careful to scale this appropriately --
\lstinline|\usepackage[scaled=0.88]{beraserif}| and
\lstinline|\usepackage[scaled=0.85]{berasans}|, and
\lstinline|usepackage[scaled=0.84]{beramono}|.
\begin{note}
I sometimes use Bera Sans for extended admonitions like this one so that they
can quickly be separated from the text.
\end{note}
\end{document}
Adobe Reader.app/Contents/Resources/Resource/Fontor similar (or search theAdobe Reader.appdirectory for*.otffiles). You have to add the fonts manually to Font Book to use them, but the font files are there and usable. – Philipp Jan 24 '11 at 13:21