In Source Insight, global variable show up in a different colour. Is there a way to accomplish this in vi (or any other free editor)? 
Highlight global variable in vim (or any other free editor)
text editor;vim
It may not be quite what you want, but you could have a look at my TagHighlight plugin for Vim.Assuming your chosen language is supported by it (or at least by ctags: it's quite easy to add new languages), this plugin will use ctags to parse your source tree and identify a lot of extra keyword types (including global variables, which will be highlighted in the GlobalVariable style, so you'll need to define this if your colour scheme doesn't do so already).