C# Signature:

[DllImport("user32.dll", SetLastError=true)]
static extern void SwitchToThisWindow(IntPtr hWnd, bool fAltTab);

VB Signature:

Declare Sub SwitchToThisWindow Lib "user32.dll" ( _
   hWnd As IntPtr, fAltTab As Boolean)

Notes:

Set fAltTab to true if you want it to work well for switching between applications. Otherwise won't work if application you're switching to is minimized.

Note that this function is deprecated and not intended for general use according to MSDN.

Tips & Tricks:

Please add some!

Sample Code:

    static void Main()
    {
        int nIndex;

        Process   procCurrent = Process.GetCurrentProcess();
        Process[] procProgram = Process.GetProcessesByName("Foo");

        /* check if program is already running */
        if(procProgram.Length > 1)
        {
            for(nIndex = 0; nIndex < procProgram.Length; nIndex++)
            {
                /* switch to the other instance and let this one die */
                if(procProgram[nIndex].Id != procCurrent.Id)
                    SwitchToThisWindow(procProgram[nIndex].MainWindowHandle, true);
            }
        }
        else
        {
            /* enable visual style, most commonly associated with the XP operating system */
            Application.EnableVisualStyles();
            Application.DoEvents();
            Application.Run(new frmFoo());
        }
    }

Alternative Managed API:

Do you know one? Please contribute it!