Parallelogram symbol

using xelatex (or lualatex):

\documentclass{article}

\usepackage{unicode-math}
\setmathfont{xits-math.otf}

\begin{document}

\[ a ^^^^25b1  b \]


\Large
\[ a ^^^^25b1  b \]

\end{document}

You don’t need TikZ, you can use PGF (just a little lighter), or if you don’t need rounded line caps/joins (or if you know how to activate them for \rules), a few rules suffice.

Code

\documentclass[varwidth]{standalone}
\usepackage{graphicx,amsmath,amssymb,pgf}
\newcommand*{\parallelogramm}{%
  \rlap{\rotatebox{-30}{\rule[.05ex]{.4pt}{.77em}}}%
  \kern.04em%
  \rlap{\kern.36em\raisebox{0.649519052835em}{\rule{.6em}{.4pt}}}%
  \rule{.6em}{.4pt}\kern-.04em%
  \rotatebox{-30}{\rule[.05ex]{.4pt}{.77em}}}
\newcommand*{\Parallelogramm}[1][]{%
  \pgfpicture\pgfsetroundjoin
    \pgftransformxslant{.6}%
    \pgfpathrectangle{\pgfpointorigin}{\pgfpoint{.60em}{.65em}}%
    \pgfusepath{stroke,#1}%
  \endpgfpicture}
\begin{document}
$\square \triangle$%
\parallelogramm
\Parallelogramm
\Parallelogramm[fill]
\end{document}

Output

With PSTricks just for fun as usual!

\documentclass{article}

\usepackage[pdf]{pstricks}


\newcommand\parallelogram[1][2]{%
    \psset{unit=#1pt}
    \begin{pspicture}(4,3)
        \pspolygon(0,0)(3,0)(4,3)(1,3)
    \end{pspicture}}

\begin{document}
This is a parallelogram \parallelogram. Is it cool? Yes! How about the bigger one \parallelogram[10]? Is it cooler? Yes!
\end{document}