frown and mathop/stackrel/overset

You may try \wideparen from the yhmath package:

\usepackage{yhmath}
...
$\wideparen{MM'}$

In case you don't want to change the largesymbol font, it's possible to load only the \wideparen accent:

\documentclass{article}
\DeclareSymbolFont{yhlargesymbols}{OMX}{yhex}{m}{n}
\DeclareMathAccent{\wideparen}{\mathord}{yhlargesymbols}{"F3}

\begin{document}
$\wideparen{MM'}$
\end{document}

You can use arcs package for this. It provides the \overarc command. But this package seems to be broken on my computer. :(

You can also use \wideparen provided by yhmath font package. yhmath is compatible with default CM fonts.

\documentclass{article}
\usepackage{yhmath}

\begin{document}
$\wideparen{MM'}$
\end{document}

MnSymbol package, mathdesign package, and maybe some other math font packages, also provide \wideparen. It would be great if you use MinionPro with MnSymbol, or Utopia with mathdesign fonts.

For example:

\documentclass{article}
\usepackage[utopia]{mathdesign}

\begin{document}
$\wideparen{MM'}$
\end{document}

Although this question is quite old, here’s a solution using TikZ:

\documentclass{article}

\usepackage{parskip}

\usepackage{tikz}
   \usetikzlibrary{calc}

\newcommand{\overbow}[1]{
   \tikz [baseline = (N.base), every node/.style={}] {
      \node [inner sep = 0pt] (N) {$#1$};
      \draw [line width = 0.4pt] plot [smooth, tension=1.3] coordinates {
         ($(N.north west) + (0.1ex,0)$)
         ($(N.north)      + (0,0.5ex)$)
         ($(N.north east) + (0,0)$)
      };
   }
}

\begin{document}
$ABC$ $\overline{ABC}$ $\overbow{ABC}$

$\overline{AB}$ $\overbow{AB}$

$\overline{MC}$ $\overbow{MC}$

$\overline{HA}$ $\overbow{HA}$

$\overline{AH}$ $\overbow{AH}$

$A \overline{A} \overbow{A}$

$xgfx \overline{gf}x \overbow{gf}x$
\end{document}

To use it with other fonts, the line width and shifting values may have to be adapted.