My number one GExpert is Grep Search.
Close second is the Procedure List window.
Also very handy are the various keyboard shortcuts,
and the toolbar embedded in the editor - I have a few
buttons on there; like CPU-view, Project Manager and
the selection-tools as a dropdown (like sort selection).
Sometimes I use the Clipboard History window.
And even more rarely the ASCII windows.
Everything else I don't touch really.
I know the procedure-list could also be found in the structure-pane of Delphi 2009, but somehow the fact that I can press Ctrl+G to get a window with quick filtering and preview capabilities is more productive for me.
Same thing for grep - the GExperts' version is way more powerfull than the standard Delphi search function.
If these two (and maybe the clipboard history) where embedded in standard Delphi, I probably wouldn't install GExperts anymore.
But on the other hand : I do like the fact that these solutions are available as open source - this allowed me for example to add section-filtering to the grep-search, which otherwise wouldn't have been possible for me...