A Simple Letter (letter.cls)
\documentclass[12pt]{letter}
\usepackage[a4paper]{geometry}
\usepackage[british]{babel}
\begin{document}
\begin{letter}{Mrs Mabel Canary\\24 The Street\\
Some Village\\Some Town\\Noshire\\AB1 2YZ}
\opening{Dear Mrs~Canary}
This is an imaginary letter.
This is the second paragraph of the letter.
\closing{Yours sincerely}
\ps PS: this is a postscript.
\encl{Photocopy of something interesting\\
Photocopy of something rather dull}
\cc{Prof Important Person\\Dr Bor Ing}
\end{letter}
\end{document}
Download letter-sample.tex or letter-sample.pdf.