FILLET is an old favorite and has been updated many times throughout the years...
Being a "sparky" (that's what they call us electrical types) there are many right-angle or zero-radius corners.
Fillet with a radius of 0.0 has been a great time saver since the command was introduced in Version 1.4 (Release 4) October 1983.
Thanks to Shaan Hurley's Unofficial AutoCAD History Pages here is the 411 on FILLET.
Fillet was updated to include polylines in Version 2.1 (Release 6) May 1985 .
Fillet was then made to work with arcs and circles Version 2.5 (Release 7) June 1986 .
In Release 11 October 1990 Filleting two Lines that cross, AutoCAD now retains the "picked" line segments. In previous versions, the shorter ends were trimmed no matter which points you used to select the lines.
In Release 13 November 1994 New option: Trim. The extrusion direction of the object you select no longer needs to be parallel to the Z axis of the current UCS. R13 <- YIKES!
Somewhere in here FILLET was added to work with 3D SOLIDS... not sure, but although clunky it works on them.
And then in AutoCAD 2000i July 2000 some valuable user feedback and requests added Fillet and Chamfer multiple polylines, Change Fillet Radius without Exiting Fillet Command.
Listed as "FILLET - Rounds and fillets the edges of objects " as a change ??? in AutoCAD 2006 March 2005.
I am not sure which release introduced the ability to use a crossing to select two objects to select to be FILLETed, nor do I remember when selecting two parallel lines/plines would produce a rounded endcap (great for slots, or in the electrical discipline cable shielding).
At last, where the Multiple sub-option was added to FILLET (keeps the command looping)... no doubt a very small enhancement that leads to huge time savings (remember right-clicking, banging the spacebar or enter key to repeat FILLETS? I do :-(
Another improvement or feature is that a user can fillet a PLINE to a non-pline object and it fillets and joins all in the FILLET commands operation.
One last improvement, again I can't remember when it was added, is that a user can set a radius and leave that as default (e.g.: 1.5 units) and while using the FILLET command you can hold down the SHIFT key to still get a zero radius fillet... another small but welcomed feature.
If you have more fillet tips, tricks or comments... please email me with FILLET in the subject line... or leave a comment here.