Is there a plugin/option that allows intellisense to auto complete words that exists in the document/solution that's currently opened in Visual Studio 2008?
If you have your solution with a main project and another secondary project on it. You should reference the secondary project in the main project and then Visual Studio should get automatically all public Classes, Functions and Methods.
If you still don't get it you can always make an intellisense help file using sandCastle Help File Builder: For Sandcastle you can go to the FAQ and search for more info.
I hope this helps you :)
I think IntelliSense should do this, try saving and compiling the file first to see if it happens then. If not, you will either need an extension that is capable do that on-the-fly or check out with a trial version of Visual Studio 2010 if it happens there.