Evaluation of Differentiation and Integration

The \big| or \Big| symbols work quite well

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[ \int_a^b x^2\;\mathrm{d}x= \tfrac{1}{3} x^3 \Big|_a^b \]
\end{document}

The \left - \right construct gives you an expandable evaluation symbol:

\documentclass[12pt]{article}
\usepackage{amsmath}

\DeclareMathOperator{\di}{d\!}
\newcommand*\Eval[3]{\left.#1\right\rvert_{#2}^{#3}}

\begin{document}

\[
  \int_{a}^{b}x\di x = \Eval{\dfrac{1}{2}x^{2}}{a}{b}
\]

\[
  \int_{a}^{b}\di x = \Eval{x}{a}{b}
\]

\end{document}

EDIT: I modified the code following the comment by Ryan Reich.

The commath package has the \eval command for this purpose.