How can I insert a subject line in a letter?

I know an answer has already been provided, but I would like to propose an alternative solution that I believe is more elegant. I would include the subject as the last line of the to-address, I also believe this is sometimes done intentionally so the subject is visible in windowed envelopes.

This would result in

...
\begin{letter}{Name\\Street\\City\\%
   \textbf{Subject: subject}%
}
...
\end{letter}
...

There's no specific command for subjects in letter style, so do something like this:

\bigskip
\textbf{Subject line}

\bigskip\noindent
First paragraph of text...

The answer where the subject is part of the address is not that elegant if I say so myself. Although if you place a tilde (~) in as a line, you get an empty line after the address and it looks nicer.

...
\begin{letter}{Name \\ street \\ city \\ ~ \\ subject}

....