Seeking ArcGIS Desktop Tips and Tricks

This question has been converted to Community Wiki and wiki locked because it is an example of a question that seeks a list of answers and appears to be popular enough to protect it from closure. It should be treated as a special case and should not be viewed as the type of question that is encouraged on this, or any Stack Exchange site, but if you wish to contribute more content to it then feel free to do so by editing this answer.

---

Even some GIS Administrators do not know about this one.

C:\Program Files (x86)\ArcGIS\Desktop10.0\Utilities (on Windows)

Advanced ArcMap Settings tool

I need to update this http://mapperz.blogspot.com/2007/02/esri-arcgis-92-tips-and-tricks.html (just don't the have time, anyone can update - will re-post with full credit)

here is a start for rel 10... Desktop 10 tips and tricks

In arcpy you can get access to the geoJSON representation of any geometry through the hidden __geo_interface__ method. For example:

arcpy.PointGeometry(arcpy.Point(5,5)).__geo_interface__

Which nicely outputs:

{'coordinates': (5.0, 5.0), 'type': 'Point'}

*All credit to Bruce Harold at ESRI Redlands for detailing this originally.