10 AutoCAD commands users need everyday

1. CONVERTPOLY (R14-R2010) - This undocumented command will allow you to convert "heavy" polylines to LWPOLYLINE's and vice versa. "Heavy" polylines are the pre-R14 version. They still have uses today, and the PLINETYPE system variable controls what gets created by default.
2. DBLIST (Version 2.0 - R2010) - This command will list every entity in the drawing. Useful when you think you have some entities that are not showing up. You can erase everything visible and then run DBLIST to see what else is still there.
3. XOPEN (R2004-R2010) - Use this command to pick an external reference to open it. You don't need to know the name of the xref.
4. TRACE (Prior to Version 1.4-R2010) - This is a really old one. It creates a TRACE object, which is sort of looks like a polyline, but each segment behaves more like a SOLID entity, which brings us to...
5. SOLID (Prior to Version 2.0-R2010) - This command creates a 2D solid entity with three or four sides. Before solid hatch was introduced, this was about the only way to get a solid fill, and you had to do some tricks around curves.
6. SETBYLAYER (R2008-R2010) - Pretty self-explanitory. Use this to set certain object properties to BYLAYER, including Color, Linetype, Lineweight, and Material.
7. REVERSE (R2010) - Used to reverse the direction of a line, polyline, spline or helix.
8. TPNAVIGATE (R2008-R2010) - Used to display a certain tool palette. If the tool palette is not visible, this command will make it visible also.
9. SAVEIMG (R14-R2010) - If you just need a quick screen capture of your AutoCAD display, this command can do it. Plotting to Raster using the appropriate driver will usually result in better output with more options, but if you just need a quick file, SAVEIMG can do it.
10. TSPACEINVADERS (R??-R2010) - This is an undocumented command. Essentially, you perform a selection and it will show you TEXT and MTEXT that has entities blocking it. It doesn't do the cleanup, but in a crowded drawing it can help you identity text under linework, etc.