I just came from Visual Studio but I'm pretty sure we had this in MonoDevelop that was shipped with older versions of Unity.
I want to click on a variable/class/method/etc. and have it - and all of its usage in the file (syntax aware) - highlighted.
I'm using the newer version of MonoDevelop 4.0.1 that is being shipped with Unity 4.3.
I have enabled Syntax Highlighting in the preferences, but nothing. Is it somewhere else?
Preferences > Text Editor > Markers And Rulers
Then Check the check box "Highlight identifier reference".
That should do the work.