Return to an already open application when a user tries to open a new instance

I found this code to be useful. It does the detection and optional activation of an existing application:

http://www.codeproject.com/KB/cs/cssingprocess.aspx

If you're using .NET, this seems easier and more straightforward using build-in .NET functionality:

The Weekly Source Code 31- Single Instance WinForms and Microsoft.VisualBasic.dll