Lets create Outlook 2007 Add-In using Visual Studio 2005.
If you have not installed VSTO then
Download Microsoft Visual Studio 2005 Tools for Office Second Edition Runtime (VSTO 2005 SE) (x86).Start IDE.
Click on File, New project

Select Outlook Add-In and Click on OK.
you will find some code like this:
Public Class ThisAddIn Private WithEvents btn1 As Office.CommandBarButton
Private Sub ThisAddIn_Startup(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Startup
End Sub
Private Sub ThisAddIn_Shutdown(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Shutdown
End Sub
End Class
Now add following code in StartUp event:
Dim explorer As Outlook.Explorer = Me.Application.ActiveExplorer() If explorer IsNot Nothing Then
Dim bar As Office.CommandBar = explorer.CommandBars.Add("saAction Bar", Temporary:=True)
bar.Visible = True
btn1 = bar.Controls.Add(Office.MsoControlType.msoControlButton, Temporary:=True) btn1.Caption = "saAction Button"
btn1.Tag = "myOutlookAddin1.btn1"
btn1.Style = Office.MsoButtonStyle.msoButtonCaption
End If
And declare Button click event any where in the class.
Private Sub Btn1_Click(ByVal ctrl As Office.CommandBarButton, ByRef cancelDefault As Boolean) Handles btn1.Click MsgBox("Hi! this is VSTO")
End Sub
Done.
Now, Just run the project. Press F5, Outlook will automatically start and It will show a button there, and click on that button. See below "saAction Button" and messagebox.

Outlook is a very big application and using VSTO, you can utilize it's functions.