How to make Visual Studio 2010 highlight identical, selected words in editor ?

There is an extension on Microsofts Visual Studio Gallery named "Highlight all occurrences of selected word" which promises what you're looking for.

Umm, this is a built in feature.

http://blogs.msdn.com/b/zainnab/archive/2010/01/18/how-to-use-reference-highlighting.aspx

http://msdn.microsoft.com/en-us/library/ee349251.aspx