\documentclass{article}
\usepackage{xcolor}
\usepackage{diagbox}
\begin{document}

\par\noindent
\begin{tabular}{l|c|}
  \hline
NW & \diagbox[dir=NW]{A}{B} \\\hline
SE & \diagbox[dir=SE]{A}{B} \\\hline
SW & \diagbox[dir=SW]{A}{B} \\\hline
NE & \diagbox[dir=NE]{A}{B} \\\hline
\end{tabular}
\par\noindent
\begin{tabular}{l|c|}
  \hline
NW & \diagbox[dir=NW]{A}{M}{B} \\\hline
SE & \diagbox[dir=SE]{A}{M}{B} \\\hline
SW & \diagbox[dir=SW]{A}{M}{B} \\\hline
NE & \diagbox[dir=NE]{A}{M}{B} \\\hline
\end{tabular}

\par\noindent
\begin{tabular}{l|c|}
  \hline
NW & \diagbox[dir=NW]{Word\\A}{Word\\M}{Word\\B} \\\hline
SE & \diagbox[dir=SE]{Word\\A}{Word\\M}{Word\\B} \\\hline
SW & \diagbox[dir=SW]{Word\\A}{Word\\M}{Word\\B} \\\hline
NE & \diagbox[dir=NE]{Word\\A}{Word\\M}{Word\\B} \\\hline
\end{tabular}

Slashbox compatibility
\par\noindent
\begin{tabular}{|c|c|c|} \hline
\slashbox[2cm]{num}{alpha}
& A & B \\\hline
1 & A1 & B1 \\\hline
2 & A2 & B2 \\\hline
\end{tabular}
\par\noindent
\begin{tabular}{|c|c|c|} \hline
\backslashbox[2cm]{num}{alpha}
& A & B \\\hline
1 & A1 & B1 \\\hline
2 & A2 & B2 \\\hline
\end{tabular}

More exotic options
\par\noindent
\begin{tabular}{l|c|}
  \hline
\diagbox[dir=NW,width=1in,height=1in,linewidth=2pt,linecolor=blue,font=\itshape]{Head\\A}{Head\\B} & B\\\hline
A & AB
\end{tabular}
\begin{tabular}{l|c|}
  \hline
\diagbox[dir=NW,innerwidth=1in,linewidth=2pt,linecolor=blue,font=\itshape]{Head\\A}{Head\\B} & B\\\hline
A & AB
\end{tabular}

\end{document}