I've been doing some C# development with the Excel object model and when you look at Excel objects in the object browser or in intellisense there are two major problems you run into:

1) Methods that would be hidden in VBA are shown in the object browser when using C#.  You really probably shouldn't be calling these hidden methods.

2) It is hard to tell which parameters are required and which are optional.  I mean, you can usually guess that if it the parameter is of type object, it is probably optional and you can pass a Type.Missing to it, but you can't know for sure.

Which is why I've been using officewiki.org as a tool to help me with these problems.  Officewiki.org only shows you the methods and properties that are not hidden.  It also tells you explicitly which parameters are optional.

It's really easy to type in a URL to get right to the object you want.  Here are some examples:

Workbooks object: http://www.officewiki.org/default.aspx/Excel.Workbooks

Workbooks.Count property: http://www.officewiki.org/default.aspx/Excel.WorkbooksCountProperty

Workbooks.Add method: http://www.officewiki.org/default.aspx/Excel.WorkbooksAddMethod

Outlook Inspector object: http://www.officewiki.org/default.aspx/Outlook.Inspector

and so forth.