<p>I have a set of calculation methods sitting in a .Net DLL.  I would like to make those methods available to Excel (2003+) users so they can use them in their spreadsheets.</p>

<p>For example, my .net method:</p>

<pre><code>public double CalculateSomethingReallyComplex(double a, double b) {...}
</code></pre>

<p>I would like enable them to call this method just by typing a formula in a random cell:</p>

<pre><code>=CalculateSomethingReallyComplex(A1, B1)
</code></pre>

<p>What would be the best way to accomplish this?</p>

There are two methods - you can used Visual Studio Tools for Office (VSTO):
http://blogs.msdn.com/pstubbs/archive/2004/12/31/344964.aspx
or you can use COM:
http://blogs.msdn.com/eric_carter/archive/2004/12/01/273127.aspx
I'm not sure if the VSTO method would work in older versions of Excel, but the COM method should work fine.