A couple times a month, I check out the various tips and tricks CW posts on Stack Overflow and always learn something new.
I went back and forth as to whether to pin this question to a specific program of a specific, major version of GIS software. Given the amount of change to the ux, I think ArcGIS Desktop 10 should get its own Tips and Tricks question and will be a great resource for new ArcGIS users and ArcGIS 10 users upgrading from 9.3.
Hope to see some good tips that aren't already covered in the ESRI ArcGIS Desktop Tips and Shortcuts.
ArcMap
If the File menu is slow to open: open Customize window, navigate to the file menu, and delete the { Sign in } menu item.
Revert to old(classic) snapping environment: see Question on how to use the editor tool and snapping in ArcGIS 10